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Vol.8,No.2 November 2007
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Vol. 8, No. 2
November 2007
THE INTERNATIONAL
HYDROGRAPHIC REVIEW
SPONSORED BY THE INTERNATIONAL HYDROGRAPHIC ORGANISATION
Published by:
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
4

INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Editorial
Contents
By Adam J. Kerr, Editor 7
Articles
Continental Shelf Submissions: an Updated Record
By Ron Macnab, Geological Survey of Canada (Retired), (Canada) 9
Predicting Sand Wave Dynamics
On the Netherlands Continental Shelf
By Roderik Lindenbergh, Delft University of Technology; Leendert Dorst,
Netherlands Hydrographic Service and University of Twente; Hans Wüst,
Ministry of Transport, Public Works and Water Management, Centre for Transport
and Navigation, and Peter Menting, Fugro-Inpark B.V. (The Netherlands) 25
Encoding AIS Binary Messages in XML Format
for Providing Hydrographic-related Information
Kurt Schwehr and Lee Alexander, Center for Coastal and Ocean Mapping/Joint
Hydrographic Center, University of New Hampshire, Durham, New Hampshire (USA) 37
GPS Techniques in Tidal Modelling
Dave Mann, Survey Support Manager, Gardline Geosurvey Ltd (UK) 59
Notes
The WEND Concept for a Worldwide ENC Database - Past or Future
A Review of Progress and a Look to the Future
Horst Hecht, Federal Maritime and Hydrographic Agency (BSH), Hamburg/
Rostock (Germany); Abri Kampfer, Hydrographic Office, Cape Town (Republic of
South Africa); Lee Alexander, CCOM-JHC, University of New Hampshire, Durham,
New Hampshire (USA) 73
Relationship of Marine Information Overlays (MIOs) to Current/Future
IHO Standards
By Dr. Lee Alexander, Chair and Michel Huet, Secretary, IHO-IEC Harmonization
Group on Marine Information Overlays (USA) 80
Spatial Solution To Determine A Trigonometric Point Of High Precision
By S.Ackermann, Università degli studi di Napoli "Parthenope" and A.Vassallo,
Istituto Idrografico della Marina (Italy) 83
Instructions for Contributors
90
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
The International Hydrographic Review
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Editor-in-chief- : Adam J. Kerr
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Editorial
This issue touches on a broad range of hydrographic interests but all of the topics
covered are important to the development of the profession. Approaches to tidal
modelling using GPS comes to us from a commercial source. In this the particular
difficulties of obtaining tidal heights in the offshore are discussed. Reference to
GPS brings with it the matter of choice of vertical datums. Also concerned with
the problem of obtaining precise depth measurement is a paper discussing the
prediction of sandwave dynamics in the shallow North Sea off the Netherlands.
To all those countries with a mobile sedimentary seafloor a knowledge of how the
sea floor topography is actually changing is critical. Particularly where shipping is
passing through areas with very marginal underkeel clearance. The Netherlands has
been most active in developing models that will assist in the prediction of these
changes and this paper discusses different approaches that are being taken to
improve the predictions.
A particularly relevant report on the status of claims to extending the continental
shelf beyond 200 nautical miles has been prepared by a Canadian specialist. As
the time limit for filing claims with the Continental Shelf Commission approaches,
the Commission is being pressed into greater activity to review and comment upon
the claims. A particular problem is that some claims require agreement between the
claims of neighbouring states and the time taken for that procedure may overwhelm
the time allowed for filing the claims. Claims around the Antarctic cause particular
difficulties due to the uncertain question of sovereignty of the land itself. In the Arotic
large claims, based on the claim that large undersea features are part of national
natural prolongation threaten to make all Ambassador Pardo's fears that there will
be little left for a "common heritage" come true.
In various ways ENCs and ECDIS are the subject of a paper and several notes. Encoding
AIS ( Automatic Information System) messages is discussed as this information
is increasingly used as part of a total navigation system. Then there is the ongoing
examination of just how information other than basic chart information can be
added to the total digital information provided. Finally an important note is provided
on the WEND Concept. This idea for providing ENCs on a global basis has long been
pursued by the IHO. A much slower than expected development of ENCs by national
hydrographic offices has been a subject of considerable criticism. Today there are
strongly opposing views on how things stand at the matter but the WEND is felt to
be the framework within which this network of official data should be provided.
Adam J. Kerr, Editor
7

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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Article
Continental Shelf Submissions: an Updated Record
By Ron Macnab, Geological Survey of Canada (Retired), (Canada)
Abstracts
To date, nine coastal states have presented a total of eight submissions
for continental shelf extensions beyond their 200 nautical mile
limits. This paper summarizes the scopes of those submissions and the stages they
have attained in their examinations by the Commission on the Limits of the Continental
Shelf. The paper also identifies the members of the three Commissions that
have been elected since 1997, and of the seven subcommissions that have been
established since 2001 for the purpose of reviewing individual submissions.
Résumé
A ce jour, neuf Etats côtiers ont présenté huit soumissions au total
pour des extensions du plateau continental au-delà de la limite des
200 milles marins. Le présent article résume la portée de ces soumissions et les
stades atteints dans I'examen par la Commission sur les limites du plateau
continental. Cet article identifie également les membres des trois Commissions qui
ont été élus depuis 1997, et les sept sous-commissions établies depuis 2001 dans
le but de passer en revue chaque soumission.
Resumen
Hoy, nueve Estados costeros han presentado un total de ocho
propuestas sumisiones para la extensión de la plataforma
continental mas allá del límite de las 200 millas náuticas. Este artículo resume los
aspectos de esas sumisiones y las etapas que han logrado en sus exámenes por
la Comisión de Límites de la Plataforma Continental. El artículo también identifica
los miembros de las tres Comisiones que han sido elegidos desde 1997, y de las
siete subcomisiones que han sido establecidas desde el 2001 con el propósito de
revisar las sumisiones individuales.
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INTERNATIONAL HYDROGRAPHIC REVIEW
This report outlines the scope and status of the first
eight continental shelf submissions to be presented
to the Commission on the Limits of the Continental
Shelf (CLCS), consisting of: five single submissions
from Russia, Brazil, Australia, New Zealand,
and Norway; two partial submissions from Ireland
and France, and one joint submission from France,
Ireland, Spain, and the United Kingdom. It also includes
a commentary concerning the compositions
and functions of the CLCS, and of the subcommissions
that have been established so far to examine
the submissions.
submissions are not made public, nor are the deliberations
of the CLCS concerning those submissions.
In certain cases some of that information can be
gleaned through unofficial channels, but for the most
part interested parties must be satisfied with material
of a more limited nature that is posted on the
website of the United Nation's Division of Ocean Affairs
and the Law of the Sea (DOALOS): http://www.
un.org/Depts/los/clcs_new/clcs_home.htm. Most
of the information in this presentation is derived
from that official source.
Information concerning the first four submissions
was described in an earlier report, which also discussed
the compositions of the CLCS and its subcommissions
(Macnab and Parson, 2006). Portions
of that earlier paper are repeated here for the sake
of completeness.
In general, detailed contents of continental shelf
An Overview of Past and Current
Submissions
As of this writing, nine coastal States have presented
eight continental shelf submissions for consideration
by the Commission on the Limits of the Continental
Shelf (CLCS). Their submission dates are listed in
Table 1.
Figure 1: Shaded areas show the locations of the continental shelf extensions sought by Russia in the Barents Sea and in
the central Arctic Ocean. The Russian 200 nautical mile limit is portrayed by two line colours - solid red, and red & yellow
combined. The double black line is a provisional outer limit of the Russian continental shelf, its final position subject to negotiation
with neighbour states. Other components seen in this figure represent elements that figured in the development
of the Russian claim. Source: website of the UN Division of Ocean Affairs and the Law of the Sea (DOALOS).
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INTERNATIONAL HYDROGRAPHIC REVIEW
State
Russian Federation
Brazil
Australia
Ireland (partial)
New Zealand
France, Ireland, Spain, UK
(joint)
Norway
France (partial)
Submission date
2001
2004
2004
2005
2006
2006
2006
2007
The submissions for Russia, Brazil, and Ireland have
been subjected to a full review by the CLCS, which
issued outer limit recommendations for Russia in
2002, and for Brazil and Ireland in 2007. The Australian,
New Zealand, and Norwegian submissions
are still undergoing review by subcommissions of
the CLCS, as is the joint submission from France,
Ireland, Spain, and the UK. The tasks of these subcommissions
are to examine submissions and to
draft recommendations for review by the Commission
at large. As of this writing, a subcommission to
examine France's partial submission has yet to be
established.
The Russian Submission
This submission (United Nations, 2001a) specified
extended continental shelf areas in four distinct regions:
two in the Arctic, two in the northwest Pacific
(portrayed in Figures 1 and 2, respectively). It was
Figure 2: Shaded areas show the locations of the continental shelf extensions sought by Russia in the Sea of Okhotsk and
in the Bering Sea. The yellow line portrays the proposed outer limit of the juridical continental shelf of the Russian Federation.
The red and blue lines indicate the 200 nautical mile limits of Russia and the USA, respectively. The dashed black line
represents the delimitation of maritime zones defined in 1990 by agreement between the former USSR and the USA. Other
components seen in this figure represent elements that figured in the development of the Russian claim. Source: website
of the UN Division of Ocean Affairs and the Law of the Sea (DOALOS).
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INTERNATIONAL HYDROGRAPHIC REVIEW
presented to the UN Secretary-General in 2001, four
years after Russia's ratification of UNCLOS in 1997,
and eight years ahead of the 2009 deadline. It has
been suggested that the early submission date
was prompted in large part by circumstances and
priorities within the Russian Government (Skaridov,
2003).
The DOALOS website contains the following information
that is specific to this submission:
1. Press Release SEA/1726 dated December 2 1 ,
2001;
2. an unofficial English translation of an Executive
Summary which consists of four pages containing
lists of geographic coordinates accompanied
by explanatory notes, three maps, and one page
of map captions; and
3. a Statement delivered by a senior member of
the Russian deputation during the presentation
of the Russian submission to the Commission
(United Nations, 2002a).
Five States (Canada, Denmark, Japan, Norway, and
the United States) responded with communications
that addressed several aspects of the submission:
the difficulty of assessing the proposed outer limits
given the information at hand; problems arising from
overlapping jurisdictions or questionable baselines;
and the geological and tectonic interpretations that
underpinned the proposed outer limits in the central
Arctic (United Nations, 2001b). The latter concerns
reflect the many uncertainties that prevail in the Arctic
geoscientific community with regard to the tectonic
history and framework of the Amerasia Basin,
which lies between North America and Eurasia: there
is still no broad consensus on which scenario best
describes the opening of that Basin, and whether or
not the geological natures of prominent seabed elevations
such as the Lomonosov and Alpha-Mendeleev
Ridges qualify them as 'natural prolongations' of the
landmasses of adjacent coastal States.
In its recommendations (United Nations, 2002b;
paragraphs 38-41), the CLCS expressed no reservations
over proposed continental shelf extensions in
the Bering and Barents Seas. In the Sea of Okhotsk,
the CLCS recommended a partial submission, to
be accompanied by efforts to resolve jurisdictional
issues with Japan. In the central Arctic Ocean, the
CLCS recommended a revised submission.
For a more expansive discussion of the Russian sub-
Figure 3: The light green shading represents the extent
of Brazil's Exclusive Economic Zone. The dark green areas
portray the continental shelf extensions that are described
in the 2005 Addendum to the Brazilian submission. In the
southern area, the extended continental shelf closes the
gap between two EEZ regions: one generated by the mainland,
the other by the Martin Vaz Islands. Source: website
of the UN Division of Ocean Affairs and the Law of the Sea
(DOALOS).
mission, of the reactions from other States, and of
the Commission's recommendations, the reader is
referred to Macnab, 2003.
In 2003, Russia responded to the CLCS recommendations
by organizing an international conference in
St. Petersburg (Ministry of Natural Resources, 2003).
This gathering featured over thirty presentations by
Russian and non-Russian speakers who addressed
an array of geoscientific topics that were relevant to
the implementation of Article 76 in the Arctic.
Of particular interest in the St. Petersburg gathering
were concluding presentations by two senior Russian
functionaries: speaking in a personal capacity, Y. Ka-
12

INTERNATIONAL HYDROGRAPHIC REVIEW
zmin outlined the Russian Federation's reservations
concerning the validity of the CLCS recommendations;
I. Glumov spoke about his Ministry's intention
to mount a new round of field work in the Arctic, for
the purpose of obtaining additional data that would
counter CLCS concerns.
The first phase of this additional field work was completed
during the summer of 2005, examining the
geological and tectonic linkages between the Mendeleev
Ridge and the continental margin of Siberia;
preliminary results were presented at the Fall Meeting
of the American Geophysical Union (Kaminsky
et al, 2005). Another phase of field work has been
mobilized to focus on the linkages between the Lomonosov
Ridge and the Siberian margin (Poselov et
al,2007).
The Brazilian submission
This submission (United Nations, 2004a) made a
case for extended continental shelves off the country's
northern margin and off the southern half of its
eastern margin (Figure 3). The DOALOS website provides
an eight-page Executive Summary that comprises
a cover page, a page of ship photographs,
one page of text, three page-sized maps (also posted
separately in a larger format), two pages of geographic
coordinates, and one Addendum that was
submitted in 2006.
The submission attracted only one response, from
the United States of America (United Nations
2004b), which was dismissed by the CLCS on the
grounds that it did not originate from a party that
was currently involved in a boundary dispute with
Brazil (United Nations, 2004c; paragraph 17).
In 2006, Brazil submitted an Addendum to its original
submission, citing new information that supported
a change in the proposed outer limit (United
Nations, 2006a). The sub-commission charged with
assessing this submission held several sets of deliberations
(United Nations, 2005a; United Nations,
2006b; United Nations, 2006c; and United Nations,
2007a), and delivered its recommendations to the
Commission in June 2007 (United Nations, 2007a)
Figure 4: Continental shelf extensions (pink lines) sought by Australia in ten locations off the Australian mainland, off
isolated islands, and off the Australian Antarctic Territory. Australia has requested that the CLCS defer consideration of the
Antarctic extensions for the time being, in light of the continent's unique legal and political status. Source: website of the
UN Division of Ocean Affairs and the Law of the Sea (DOALOS).
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INTERNATIONAL HYDROGRAPHIC REVIEW
submission met several times
(United Nations, 2006b;
United Nations, 2006c) and
submitted its recommendations
to the full Commission in
March-April 2007; however it
was agreed that final adoption
of those recommendations
would be deferred until the
next meeting of the Commission
(United Nations, 2007a;
paragraph 33).
Figure 5: Continental shelf extension in the Porcupine Abyssal Plain, sought by
Ireland in its partial submission. The red line denotes the outer limit of Ireland's
Exclusive Economic Zone, while the thin black line portrays the current limit of the
Irish-designated continental shelf. The extents of the Icelandic and Faeroese claims
are portrayed as well. Source: website of the UN Division of Ocean Affairs and the
Law of the Sea (DOALOS).
where they were accepted with amendments.
The Australian submission
This submission (United Nations, 2004d) identified
continental shelf extensions in ten locations off the
Australian mainland, off isolated islands, and off the
Australian Antarctic Territory (Figure 4). A detailed
and informative 49-page Executive Summary is
posted on the DOALOS website, featuring a regionby-region
overview, 21 page-sized maps (also posted
separately in a larger format), and two separate Annexes
containing lists of geographic coordinates.
An accompanying Note requests that the CLCS defer
consideration of the Antarctic extension for the
time being, taking into account the unique legal and
political status of that continent according to the provisions
of the Antarctic Treaty.
The submission attracted eight responses from other
States: Germany, India, Japan, The Netherlands,
Russia, and the USA rejected the possibility of establishing
an extended continental shelf off Antarctica,
while France and Timor-Leste declared that the recommendations
of the CLCS would not be prejudicial
to the establishment of bilateral boundaries between
themselves and Australia (United Nations, 2005b).
The sub-commission charged with assessing this
An interesting sidelight
emerged during a requested
meeting between the Commission
and the Australian deputation
in March-April 2007,
when spokesmen for the latter
declared that Australia would
seek an explanation from the
Commission if its recommendations
did not conform to that
country's expectations (United
Nations, 2007a; paragraphs
30 and 32). This declaration appears to serve notice
that the CLCS should be prepared to defend its decisions
against coastal state challenges.
The Irish Submission
This submission (United Nations, 2005c) was a partial
one, in that it proposed an extended continental
shelf in the Porcupine Abyssal Plain only (Figure
5). The eight-page Executive Summary posted on
the DOALOS website features four pages of mixed
text and figures, and a one-page Appendix containing
a list of geographic coordinates. The document
explains that boundaries with neighbouring states
north and south of this shelf extension remain under
discussion, necessitating a deferral of Article 76
work in those regions.
The proposed outer limit attracted responses from
Denmark and Iceland, declaring that the submission
and the recommendations of the CLCS were to be
considered as non-prejudicial to their own interests
in the region (United Nations, 2005d). The subcommission
charged with assessing this submission
held two sets of deliberations (United Nations,
2006b; United Nations, 2006c) and presented its
draft recommendations to the CLCS for review and
consideration. These were subsequently adopted at
14

INTERNATIONAL HYDROGRAPHIC REVIEW
the first meeting of the CLCS in 2007 (United Nations,
2007a; paragraph 37).
The New Zealand Submission
This submission (United Nations, 2006d) proposed
extended continental shelves in four regions radiating
outward from the land area of New Zealand
(Figure 6). For the present, New Zealand excludes a
prospective continental shelf adjacent to Antarctica,
but it does reserve the right to present a supplementary
submission for that region at a future date.
An eighty-page Executive Summary posted on the
DOALOS website features twenty pages of mixed text
and maps, and four Appendices listing fixed points
which comprise the outer limits of the four regions.
A corrigendum lists corrections to a number of fixed
points. A covering letter acknowledges a potential
delimitation issue with France in the area of the
Three Kings Ridge.
The proposed outer limits attracted responses from
Fiji, Japan, and France (United Nations, 2006e). Fiji
and France alluded to boundary delimitation issues
in the continental shelf areas described in New Zealand's
submission, stating that the recommendations
of the CLCS should be without prejudice to upcoming
submissions from either country. Japan declared
that it did not recognize the sovereignty of any state
over the submarine areas adjacent to Antarctica. The
sub-commission charged with assessing this submission
began its deliberations in 2006 (United Nations,
2006b; paragraph 24), and presented its preliminary
findings during the first meeting of the CLCS in 2007,
with an understanding that members would continue
to work on the submission until
the end of the term of office
of the present Commission
(United Nations, 2007a; paragraph
37).
The Joint Submission from
France, Ireland, Spain, and
the UK
Prepared 'collectively and collaboratively'
by France, Ireland,
Spain, and the United Kingdom,
this submission defines a
zone (Figure 7) seaward of the
Bay of Biscay (United Nations,
2006f). Delimitation within the
zone will be resolved by the
four parties at a later date.
An eight-page Executive Summary
posted on the DOALOS
website features two maps
and a list of fixed points which
circumscribe the outer limit of
the zone.
Figure 6: The red lines separate the four regions where New Zealand is proposing
continental shelf extensions. New Zealand also reserves the right to make a future
submission off Antarctica. Source: website of the UN Division of Ocean Affairs and
the Law of the Sea (DOALOS).
The submitting states consider
that their proposed outer
limit does not infringe upon
the interests of other coastal
states. This would appear to
be borne out by the lack of reactions
from other parties. The
subcommission charged with
assessing this submission began
its work in 2006 (United
15

INTERNATIONAL HYDROGRAPHIC REVIEW
Nations, 2006c; paragraph 34)
and met several times with
representatives of the submitting
states, who were asked
to present additional material.
This material was duly
furnished, and the subcommission
proposed to draft its final
recommendations following examination
of the new information
(United Nations, 2007a;
paragraph 40).
Figure 7: Joint submission by France, Ireland, Spain, and the United Kingdom. The
northern edge of this continental shelf segment abuts the southern edge of the
Irish segment shown in Figure 5. Source: website of the UN Division of Ocean Affairs
and the Law of the Sea (DOALOS).
The Norwegian Submission
This submission (United Nations,
2006g) proposes outer
limits in three separate areas:
the 'Loop Hole' in the Barents
Sea; the Western Nansen Basin
in the Arctic Ocean; and
the 'Banana Hole' in the Norwegian
Sea (Figure 8). Norway
Figure 8: Continental shelf limits proposed by Norway in the Barents Sea, in the Western Nansen basin, and in the Norwegian
Sea. Norway reserves the right to propose extensions in other areas. Source: website of the UN Division of Ocean
Affairs and the Law of the Sea (DOALOS).
16

INTERNATIONAL HYDROGRAPHIC REVIEW
reserves the right to make future submissions
in other areas. A 22-page Executive
Summary posted on the DOALOS
website contains six maps, two technical
figures, and an Appendix listing fixed
points which circumscribe the outer limit
of the zone.
Norway refers to current or anticipated
bilateral delimitations with neighbouring
states: with Russia in the 'Loop Hole';
with Greenland and Russia in the Western
Nansen Basin; and with Denmark,
Faroes and Iceland in 'Banana Hole' (already
agreed).
Figure 9: Continental shelf limit proposed by France off French Guiana,
in a partial submission that also proposes two extensions off New
Caledonia. This Source: website of the UN Division of Ocean Affairs and
the Law of the Sea (DOALOS).
The proposed outer limits attracted
responses from Denmark (United Nations,
2007b), Iceland (United Nations,
2007c), the Russian Federation (United
Nations, 2007d), and Spain (United Nations
2007e); the first three reactions
stated that the recommendations of the
CLCS should be without prejudice to upcoming
submissions from either country,
while the last reiterated Spain's view
that parties to the Svalbard Treaty of
Figure 10: Continental shelf limits proposed by France in two areas off New Caledonia, in a partial submission that also
proposes an extension off French Guiana. Source: website of the UN Division of Ocean Affairs and the Law of the Sea
(DOALOS).
17

INTERNATIONAL HYDROGRAPHIC REVIEW
1920 were entitled to enjoy access to the resources
of the extended continental shelf. A subcommission
was established during the first CLCS meeting of
2007 and began its review of the submission, however
it was determined that work would need to be
continued in the intersessional period, or at least until
the election of a new Commission in June 2007
(United Nations, 2007a; paragraphs 52 and 54).
The French Submission
This partial submission (United Nations, 2007f)
proposes outer limits for three continental shelf extensions:
one off French Guiana, and two off New
Caledonia (Figures 9 and 10, respectively). A 22-
page Executive Summary posted on the DOALOS
website contains four maps and three appendices.
The first two appendices consist of tables that list
the geographical coordinates of the outer limit points
off French Guiana and southeast of New Caledonia;
the third appendix consists of a simple declaration
that the outer limit off southwest New Caledonia coincides
with a bilateral limit that was established by
France and Australia in 1982.
The Executive Summary also declares that the proposed
extension off French Guiana is not subject to
any dispute with neighbouring states, while the extension
southeast of New Caledonia is the subject
of exchanges with Australia and New Zealand. As of
this writing, the submission has attracted no reaction
from any other state.
Other Submissions in Waiting
The submissions described above are expected to
be augmented by submissions from ten or so States
that have declared their intentions of completing
their preparations prior to 2009 (United Nations,
2004c; paragraph 46). In a targeted survey of state
practice in the sharing of technical information, another
nine States confirmed that they were engaged
in activities related to Article 76 (S0rensen et al,
2005). Over and above these States, there remain
an undetermined number of prospective continental
shelf claimants for whom the May 2009 submission
deadline applies.
Submissions and Subcommissions: a
Snapshot
Current Status of Submissions: a Recapitulation
Table 1 illustrates the status, at the midpoint of
Table 1: Status of the eight continental shelf submissions as of mid-2007. Also portrayed along the top edge are the terms
of the Commission on the Limits of the Continental Shelf (CLCS), and along the bottom edge the 10-year time frame that
precedes the deadline for initial submissions. The CLCS has reviewed and adopted recommendations for Russia, Brazil, and
Ireland. Russia has mounted field expeditions to acquire additional data. The CLCS has not completed its consideration of
submissions from Australia, New Zealand, and Norway, nor the joint submission from France, Ireland, Spain, and the United
Kingdom. A subcommission has yet to be established for reviewing the French submission.
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INTERNATIONAL HYDROGRAPHIC REVIEW
Table 2: Chart identifying past and present members of the Commission on the Limits of the Continental Shelf (CLCS)
along with their sponsor States, and listing those who have been appointed to the first seven subcommissions (as of this
writing, no subcommission has been established to review the French submission). Also identified are past and current
CLCS members who have rendered assistance to submitting states. A change in the membership of the Russian subcommission
was necessitated when two members did not stand for re-election in 2002; similarly, six Commission members
were not re-elected in 2007, which could necessitate changes in the memberships of six subcommissions.
2007, of the eight submissions that have been
presented so far. The CLCS has reviewed the Russian
submission, and has issued recommendations
which have prompted follow-up fieldwork by Russian
agencies to acquire additional data that is intended
to buttress their case (Kaminsky et al, 2005; Poselov
et al, 2007). Presumably this new information will
be used to formulate revisions to the existing submission,
which will then have to undergo renewed
scrutiny by the CLCS.
Meanwhile, the Brazilian and Irish recommendations
have been adopted and are under consideration
by those two submitting states. The Australian,
New Zealand, and Norwegian submissions as well
as the joint submission from France, Ireland, Spain,
and the United Kingdom, are in various stages of
review by their respective subcommissions or by the
Commission at large. As of this writing, no subcommission
has been established to review the French
submission.
Membership of the First Three Commissions and of
the First Seven Subcommissions
Table 2 illustrates the memberships of the first three
Commissions that have been elected, and of the first
seven subcommissions that have been appointed so
far. Annex II of the Convention specifies that unless
the CLCS decides otherwise, a subcommission will
consist of seven members. It will be noted that the
subcommissions for Brazil, Australia, Ireland, and
Norway actually consist of eight members: in each
instance, the eighth member is a specialist advisor
drawn from the ranks of the Commission.
The term of an individual subcommission extends
from the date of its appointment to the time that
the submitting coastal State deposits charts and
relevant information regarding its outer continental
shelf limits (United Nations, 2004e; rule 42, paragraph
2). Where a revised submission has been recommended,
as in the case of Russia, the subcommission
presumably remains on a standby status
in the expectation of resuming its examination at a
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INTERNATIONAL HYDROGRAPHIC REVIEW
later date. If a member of that subcommission becomes
unavailable for service during this interval, it
may be necessary to appoint a replacement. This in
fact was done in 2004 in the case of the Russian
subcommission, in which two of the original members
had to be replaced because they did not stand
for re-election to the CLCS in 2002 (United Nations,
2004c; paragraph 47). Similarly, four subcommission
members were not re-elected in 2007, leaving
six subcommissions under strength by one or two
members apiece; presumably it will be necessary to
consider replacing some or all of these members at
a future date.
A member of the CLCS can be appointed to more
than one subcommission (United Nations, 2004e;
rule 42, paragraph 3). A perusal of Table 2 will indicate
that this is in fact happening. As more submissions
are presented, it can be anticipated that the
CLCS will need to engage in a balancing act: (a) to
avoid overloading its members with appointments to
multiple subcommissions; (b) to ensure that each
subcommission possesses an appropriate mix of
expertise; (c) to provide each subcommission with
the necessary financial resources and support facilities;
and (d) to allow for the possibility that some
members may not be available for extended service
if the terms of their subcommissions straddle the
election of a new Commission. This will no doubt
unleash significant internal stresses and strains as
the CLCS strives to accommodate its growing workload.
At the request of the Meeting of States Parties (SP-
LOS), the CLCS has proposed a set of rules to ensure
that the anticipated flow of submissions will
be handled in the most effective way. Foremost is
the decision to limit the number of active subcommissions
to three at any one time, with a new subcommission
to be established only when an existing
subcommission has delivered its draft recommendations
to the Commission (United Nations, 2006c;
paragraphs 36-38). Additional measures have also
been proposed to increase the efficiency of the
Commission's internal operations (United Nations,
2006b; paragraphs 40-41).
Mindful of the financial burden that must be borne by
sponsoring parties (particularly developing states) in
defraying the not inconsiderable expenses incurred
by Commission members in the performance of their
official duties, the CLCS has proposed to the Meet-
ing of States Parties that such reimbursements be
funded from the regular UN budget (United Nations,
2007a, paragraphs 55-58).
Services Rendered to Submitting States by CLCS
Members
Members of the CLCS are sponsored by their respective
coastal States, but they are meant to serve in
their personal capacities, and not as national representatives.
A member of the CLCS is permitted
to advise his sponsor State on the preparation of
its continental shelf submission. Whether or not he
provides this advisory service, he cannot serve on
a subcommission that is examining his sponsor's
submission, but he is allowed to participate in the
deliberations of the full Commission concerning that
submission.
Similarly, a member of the CLCS is free to advise
any State in the preparation of its continental shelf
submission, and while he cannot serve on the subcommission
that examines that submission, he can
participate in the deliberations of the full Commission
concerning that submission.
In either of the above cases, the submitting State
must identify current member(s) of the CLCS from
whom it has received assistance. It is probably no
coincidence that of the nine states which have so
far submitted seven single submissions and one
joint submission, two-thirds are past or current
sponsors of CLCS members: while developing their
proposed outer limits, it is understandable that submitting
States would seek to have an inside look
at the workings of the Commission by sponsoring
members who could provide insights on how the review
process might impact their own submissions.
Information posted on the DOALOS website makes
no mention of CLCS members (former or current)
who might have been involved in the preparation of
the Russian, New Zealand, and French submissions,
although in the case of New Zealand, it has been
reported informally that a past Commission member
assisted with the development of that country's
proposed outer limits. Only one sponsor state was
involved in the joint submission from France, Ireland,
Spain, and the UK, but the CLCS member from
that state was recruited to render advice to all four
states.
Of the eight submissions presented so far, six have
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INTERNATIONAL HYDROGRAPHIC REVIEW
benefited from advice rendered by sponsored members
(past or current) of the CLCS: these are identified
in Table 2.
The Advantages of Sponsoring a CLCS Member
There can be little doubt that a submitting State which
sponsors a CLCS member who is also a contributor
to that State's continental shelf program enjoys a significant
advantage over a State which doesn't. This
advantage transcends the purely technical sphere,
because a CLCS member can offer unique perspectives
on the Commission's internal procedures and
dynamics. This information can be potentially helpful
to the sponsoring State in the formulation of its own
continental shelf submission, and in the development
of a strategy for its presentation.
Conceivably, a sponsored member could also have
some influence on the formulation of the Commission's
final recommendations by commenting upon
certain aspects of the subcommission's draft recommendations
while under review by the full Commission.
In a court of law, this would be akin to having a
member of the defence team participate in the closing
deliberations of the jury. This could cast doubt on
the Commission's impartiality, and potentially weaken
the credibility of its recommendations.
The factors outlined in the two preceding paragraphs
could provide legitimate grounds for non-sponsor
States to be concerned about the prospects for
manipulation of the submission process by sponsor
States - it could prove difficult and costly (if not
impossible) for a non-sponsor State to benefit from
inside knowledge, or to participate in the review of
recommendations prior to their release. This would
appear to be an area where full disclosure of the
Commission's deliberations - including sponsored
members' interventions - would be a significant contribution
to the transparency of the overall process
(Macnab, 2004). It would be desirable therefore if
the CLCS - in consultation with the Meeting of States
Parties - considered the potential inequities that
could arise from the apparent imbalance between
sponsor and non-sponsor States, and took appropriate
steps to eliminate this asymmetry.
Nearly eight years have passed since the beginning
of the first ten-year time frame for preparing and presenting
continental shelf submissions. So far, only
nine States have reached the submission stage, and
just three have received recommendations from the
CLCS. Of the remaining six states, all have presented
submissions that are partial, or where the right
is reserved to make future submissions. This raises
several questions: Does a partial submission 'stop
the clock' for a coastal state while that state constructs
the rest of its proposed outer limits Is there
a time limit for completing a partial submission Will
the remainder of a partial submission be dealt with
by the same subcommission that performed the initial
review If the composition of a subcommission
has to change during its term of office, how will the
CLCS ensure consistency in its conclusions The
record is still too scanty to suggest answers to such
questions, and to support general conclusions concerning
how the Article 76 process will unfold in the
years ahead.
However it is probably safe to point out that the Russian
submission has demonstrated what can happen
when attempting to establish an extended continental
shelf in a region - in this case the Amerasia Basin in
the Arctic Ocean - where a full understanding of the
geological framework and tectonic history remains
elusive. In such a situation, it would be prudent for
the CLCS to proceed cautiously, to seek the views of
knowledgeable specialists, and perhaps even to consider
deferring a final decision rather than assume
the role of scientific arbiter.
With the Commission entering its third term, a total
of thirty-five "experts in the field of geology, geophysics,
or hydrography" (UNCLOS Annex II) have served
or are serving in its ranks. Eight members of the first
Commission either did not re-offer, or were not reelected,
resulting in a nearly forty percent turnover of
membership between the first and second Commissions.
Six members of the second Commission did
not re-offer or were not re-elected, for a turnover of
twenty-nine percent. It is not known whether comparable
levels of turnover will occur in future elections,
nor whether they will prove sufficient to encourage
periodic and healthy renewals of the Commission's
membership while ensuring that its recommendations
remain consistent and predictable.
Conclusions
Of the thirty-five Commission members, twenty-two
have been appointed to serve on subcommissions
established to examine individual State submissions.
Seventeen of these have served or are serving
on more than one subcommission. As the May 2009
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INTERNATIONAL HYDROGRAPHIC REVIEW
deadline approaches and as additional submissions
begin to accumulate, Commission members will presumably
find themselves pressed into service on a
growing number of subcommissions.
Potentially vexing issues are the roles and influence
of Commission members during two critical stages in
the Article 76 process: (a) the preparation of submissions
by their sponsor States, and (b) the review of
draft recommendations pertaining to those submissions.
In light of the non-disclosure of Commission
proceedings, States that are not privy to the internal
workings of the Commission may well consider themselves
at a disadvantage relative to those that are.
Citations
- Commonwealth of Australia, 2004. Continental
Shelf of Australia. Executive Summary. Posted on
the website of the Division of Ocean Affairs and
Law of the Sea (DOALOS), http://www.un.org/
Depts/los/ clcs_new/submissions_files/aus04/
Documents/aus_doc_es_web_delivery.pdf
- Government of Ireland, 2005. Submission to the
Commission on the Limits of the Continental Shelf
pursuant to Article 76, paragraph 8 of the United
Nations Convention on the Law of the Sea 1982 in
respect of the are abutting the Porcupine Abyssal
Plain. Part I, Executive Summary. Posted on the
website of the Division of Ocean Affairs and Law of
the Sea (DOALOS), http://www.un.org/Depts/los/
clcs_new/submissions_files/irl05/irl_exec_sum.
pdf
- Kaminsky, VD, VA Poselov, VY Glebovsky, AV Zayonchek,
and W Butsenko, 2005. Geophysical and
Geological Study of the Transition Zone between
the Mendeleev Rise and the adjacent Siberian
Shelf: Preliminary Results (abstract only). Posted
on the website of the American Geophysical Union,
http://www.agu.org/meetings/fm05/
- Macnab, R, 2003. Russia's submission for continental
shelf extensions: the first test of UNCLOS
Article 76. EEZ International, July 2003.
- Macnab, R, 2004. The case for transparency in the
delimitation of the outer continental shelf in accordance
with UNCLOS Article 76. Ocean Development
and International Law, v. 35, n. 1, p. 1-17, 2004.
- Macnab, R. and L. Parson, 2006. Continental Shelf
Submissions: the Record to Date. Symposium on
Problems of the Outer Continental Shelf, International
Tribunal for the Law of the Sea, Hamburg,
September 25, 2005. International Journal of
Coastal and Marine Law, v 2 1 , n 6, pp 309-322.
- Ministry of Natural Resources, 2003. Morphology
and Geological Nature of Deep Seabed and Submarine
Elevations in the Arctic Basin: Controversial
Scientific Issues in Context of UNCLOS/Article 76.
Summary of presentations, St. Petersburg, June 30
-July 4, 2003.
- Poselov, V. A., A.N. Minakov, V.Y. Glebovsky, A.A.
Lickhachev, 2007. Integrated geological and geophysical
study of the Lomonosov Ridge - Siberian
Shelf transition zone (abstract). Fifth International
Conference on Arctic Margins (ICAM V), 3-5 Sept.,
2007, Tromsø, Norway.
- Skaridov, A, 2003. Russian perspectives in the
Arctic Ocean. Conference on Legal and Scientific
Aspects of Continental Shelf Limits, Reykjavik, 25-
27 June, 2003.
- Sørensen, K, C. Marcussen, and R. Macnab, 2005.
Transparency in the Article 76 process: a survey
of state practice in the sharing of technical information
pertaining to the development of the outer
continental shelf. Proceedings of the Fourth ABLOS
Conference, Monaco, October 10-12,2005. Posted
on the website of the IAG/IHO/IOC Advisory Board
on the Law of the Sea, http://www.gmat.unsw.edu.
au/ablos/
- United Nations, 2001a. Submission by the Russian
Federation. Posted on the website of the Division
of Ocean Affairs and Law of the Sea (DOALOS),
http://www.un.org/Depts/los/clcs_new/submissions_files/submission_rus.htm
- United Nations, 2001b. Reaction of States to the
submission made by the Russian Federation to the
Commission on the Limits of the Continental Shelf.
Posted on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.
un.org/Depts/los/clcs_new/submissions_files/
submission_rus.htm
- United Nations, 2002a. Statement made by the
Deputy Minister for Natural Resources of the Russian
Federation during presentation of the submission
made by the Russian Federation to the Commission,
made on 28 March 2002. UN Publication
CLCS/31, April 5, 2002. United Nations, New York.
Also posted on the website of the Division of Ocean
Affairs and Law of the Sea (DOALOS), http://daccessdds.un.org/doc/UNDOC/GEN/N02/318/60/
PDF/N0231860.pdf0penElement
- United Nations, 2002b. Report of the Secretary-General
to the Fifty-seventh session of the General Assembly
under the agenda item Oceans and the Law
of the Sea. UN Publication A/57/57/Add.1. United
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Nations, New York.
- United Nations, 2004a. Submission by Brazil. Posted
on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.un.org/
Depts/los/clcs_new/submissions_files/submission_bra.htm
- United Nations, 2004b. United States of America:
Notification regarding the submission made by Brazil
to the Commission on the Limits of the Continental
Shelf. Posted on the website of the Division
of Ocean Affairs and Law of the Sea (DOALOS),
http://www.un.org/Depts/los/clcs_new/submissions_files/bra04/clcs_02_2004Jos_usatext.pdf
- United Nations, 2004c. Statement by the Chairman
of the Commission on the Limits of the Continental
Shelf on the progress of work in the Commission.
UN Publication CLCS/42, September 14, 2004.
United Nations, New York.
- United Nations, 2004d. Submission by Australia.
Posted on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.
un.org/Depts/los/clcs_new/submissions_files/
submission_aus.htm
- United Nations, 2004e. Rules of Procedure of the
Commission on the Limits of the Continental Shelf.
UN Publication CLCS/40, July 2, 2004. United Nations,
New York.
- United Nations, 2005a. Statement by the Chairman
of the Commission on the Limits of the Continental
Shelf on the progress of work in the Commission.
UN Publication CLCS/48, October 6, 2005. United
Nations, New York.
- United Nations, 2005b. Reaction of States to the
submission made by Australia to the Commission
on the Limits of the Continental Shelf. Posted on
the website of the Division of Ocean Affairs and
Law of the Sea (DOALOS), http://www.un.org/
Depts/los/clcs_new/submissions_files/submission_aus.htm
- United Nations, 2005c. Submission by Ireland. Posted
on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.un.org/
Depts/los/clcs_new/submissions_files/submissionJrl.htm
- United Nations, 2005d. Reaction of States to the
submission made by Ireland to the Commission on
the Limits of the Continental Shelf. Posted on the
website of the Division of Ocean Affairs and Law of
the Sea (DOALOS), http://www.un.org/Depts/los/
clcs_new/submissions_files/submission_irl.htm
- United Nations, 2006a. The Brazilian submission to
the Commission on the Limits of the Continental
Shelf pursuant to Article 76 of the United Nations
Convention on the Law of the Sea; Addendum to
the Executive Summary dated 17 May 2004. Posted
on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.un.org/
Depts/los/clcs_new/submissions_files/bra04/
bra_add_executive_summary.pdf
- United Nations, 2006b. Statement by the Chairman
of the Commission on the Limits of the Continental
Shelf on the progress of work in the Commission.
UN Publication CLCS/50, May 10, 2006. United
Nations, New York.
- United Nations, 2006c. Statement by the Chairman
of the Commission on the Limits of the Continental
Shelf on the progress of work in the Commission.
UN Publication CLCS/52, October 6, 2006. United
Nations, New York.
- United Nations, 2006d. Submission by New Zealand.
Posted on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.
un.org/Depts/los/clcs_new/submissions_files/
submission_nzl.htm
- United Nations, 2006e. Reaction of States to the
submission made by New Zealand to the Commission
on the Limits of the Continental Shelf. Posted
on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.un.org/
Depts/los/clcs_new/submissions_files/submission_nzl.htm
- United Nations, 20061. Joint Submission to the
Commission on the Limits of the Continental Shelf
pursuant to Article 76, paragraph 8 of the United
Nations Convention on the Law of the Sea 1982
in respect of the area of the Celtic Sea and the
Bay of Biscay. Posted on the website of the Division
of Ocean Affairs and Law of the Sea (DOALOS),
http://www.un.org/Depts/los/clcs_new/submissions_files/frgbires06/joint_submission_executive_summary_english.pdf
- United Nations, 2006g. Continental Shelf Submission
of Norway in respect of areas in the Arctic
Ocean, the Barents Sea and the Norwegian Sea.
Posted on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.
un.org/Depts/los/clcs_new/submissions_files/
nor06/nor_exec_sum.pdf
- United Nations, 2007a. Statement by the Chairman
of the Commission on the Limits of the Continental
Shelf on the progress of work in the Commission.
UN Publication CLCS/54, April 27, 2007. United
Nations, New York.
- United Nations, 2007b. Reaction of Denmark to the
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submission made by Norway. Posted on the website
of the Division of Ocean Affairs and Law of the Sea
(DOALOS), http://www.un.org/Depts/los/clcs_
new/submissions_files/nor06/dnk07_00218.pdf
- United Nations, 2007c. Reaction of Iceland to the
submission made by Norway. Posted on the website
of the Division of Ocean Affairs and Law of
the Sea (DOALOS), http://www.un.org/Depts/los/
clcs_new/submissions_files/nor06/isl07_00223.
pdf
- United Nations, 2007d. Reaction of the Russian
Federation to the submission made by Norway.
Posted on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.
un.org/Depts/los/clcs_new/submissions_files/
nor06/rus_07_00325.pdf
- United Nations, 2007e. Reaction of Spain to the
submission made by Norway. Posted on the website
of the Division of Ocean Affairs and Law of the Sea
(DOALOS), http://www.un.org/Depts/los/clcs_
new/submissions_files/nor06/esp_0700348.pdf
United Nations, 2007f. Submission by France. Posted
on the website of the Division of Ocean Affairs
and Law of the Sea (DOALOS), http://www.un.org/
Depts/los/clcs_new/submissions_files/submission_fra.htm
Biography of the Author
Ron Macnab is a retired marine geophysicist who
wrote his first paper on UNCLOS and continental
shelf extensions in 1987. Among other affiliations,
he is a member of the American Geophysical Union
(AGU) and of the International Law Association (ILA),
where he participates in the deliberations of the
Committee on Legal Issues of the Continental Shelf
(CLIOCS). He is a past chairman of the IAG/IHO/IOC
Advisory Board on Legal and Technical Issues of the
Law of the Sea (ABLOS).
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Article
Predicting Sand Wave Dynamics
On the Netherlands Continental Shelf
By Roderik Lindenbergh, Delft University of Technology; Leendert Dorst, Netherlands
Hydrographic Service and University of Twente; Hans Wüst, Ministry of Transport, Public
Works and Water Management, Centre for Transport and Navigation, and Peter Menting,
Fugro-Inpark B.V. (The Netherlands)
Abstract
The Dutch North Sea part is mapped by two authorities. Both developed
a method for analyzing time series of bathymetric data and
predicting future sea floor depths. The Netherlands Hydrographic Service can use
deformation analysis to describe sand wave dynamics for a complete area and to
predict depth changes for individual grid points. RWS North Sea maintains a spatiotemporal
Kalman filter to estimate and predict local sea floor dynamics. We combine
both approaches to obtain a state space prediction method that incorporates sand
wave propagation.
Résumé
La représentation cartographique de la partie néerlandaise de la
mer du Nord est assurée par deux autorités qui ont mis au point une
méthode d'analyse des séries chronologiques des données bathyméthques et de
prédiction des futures profondeurs des fonds marins. Le Service Hydrographique
Néerlandais peut utiliser Tanalyse des déformations pour décrire la dynamique des
fonds mobiles pour une zone complete et pour prédire les changements de profondeurs
pour chaque point du quadrillage. RWS North Sea assure la maintenance d'un
filtre de Kalman spatio-temporel pour estimer et predire la dynamique des fonds
marins locaux. Nous combinons les deux approches pour obtenir une méthode de
prédiction de I'état de I'espace qui incorpore la propagation des fonds mobiles.
Resumen
La parte correspondiente a los Países Bajos del Mar del Norte es
cartografiada por dos autoridades. Ambas desarrollaron un método
para analizar series de tiempo de dates hidrográficos y predicción de profundidades
futuras del suelo marino. El Servicio Hidrográfico de los Países Bajos puede utilizar
análisis de deformación para describir la dinámica de las ondas de arena para una
completa área y predecir cambios de profundidades para puntos individuates de
grilla. RWS North Sea mantiene un filtro Kalman espacio-temporal para estimary
predecir la dinámica local del suelo marino. Nosotros combinamos ambas l
aproximaciones para obtener un método de predicción del estado del espacio que
incorpore la propagación de la onda de arena.
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INTERNATIONAL HYDROGRAPHIC REVIEW
1. Introduction
The North Sea is a shallow, sandy sea in Western
Europe. The Dutch part of it, the Netherlands Continental
Shelf, see Figure 1, is largely covered by
rhythmic features, like shore face connected ridges,
tidal banks and sand waves. Sand waves are rhythmic
patterns of a few meters high with wavelengths
in the order of hundreds of meters (Van Alphen and
Damoiseaux 1989). These bed forms are created by
the action of tidal and wind induced currents on the
sea floor sediment (Németh et al. 2002). At some
locations sand waves are reported to migrate with
meters a year, at other locations hardly any movement
at all is detected, (Van Dijk and Kleinhans
2005).
It is essential to have reliable and up to date information
on the depth of the Southern North Sea:
it gives access to major ports like Rotterdam and
Antwerp, while at many locations its depth is critical
and constantly changing because of migrating
bed forms. The focus of this paper is on the prediction
of future depth changes based on available
time series of depth soundings. These predictions
are used for two management decisions. The first
decision is on dredging. If a prediction indicates
that a critical depth is not sufficiently guaranteed,
a dredging project has to be scheduled. To reduce
costs, nearby predictions are taken into account to
optimize time and location of the dredging activities.
The second decision is on when to schedule a resurvey,
see Figure 2. If the depth of the sea floor in a
relatively shallow maintenance area turns out to be
changing due to e.g. sand wave dynamics, obviously
a timely new survey is required. For a currently safe
area where no changes are predicted, no new survey
has to be scheduled yet.
Several approaches exist, to obtain insight in sea
floor and especially sand wave dynamics. Important
physical insight is obtained by establishing dynamical
systems that analyze the response of a sea floor
to parameters like grain size, wave climate, water
depth and size and directions of tidal currents, e.g.
(Hulscher and Van den Brink, 2001). The echosounding
data in itself can be used to analyse the
migration of a complete sand wave field, based on
a pattern recognition approach that tracks the position
of the sand wave crests through time (Knaapen
2005, Duffy and Hughes-Clarke 2005). In this paper,
we describe, compare and combine two statistical
methods as developed independently at the Netherlands
Hydrographic Service and RWS North Sea, the
two organizations that are surveying the Dutch part
of the North Sea. Both authorities use echo sounding
observations to monitor the sea floor depth.
The Hydrographic Service is part of the Royal Netherlands
Navy and is responsible for the nautical publications
of the Netherlands Continental Shelf. In
order to design an optimal resurveying strategy, they
developed a method for analyzing time series, based
on geodetic deformation analysis, (Dorst 2004). The
core of this method is a testing procedure to determine
if the sea floor is static or if its depth is
changing over time. This procedure is applied at two
scales, for single grid-points or for a whole area. The
point-wise analysis results could be used for predictions
of future sea floor depths. Furthermore, they
estimate sand wave parameters, like sand wave
length, amplitude and location (phase), within the
area analysis. This method is designed and has
proven itself for the detection of past dynamics, for
prediction the results are not yet complete.
Figure 1: Dutch coastal bathymetry, as stored in the bathymetric
archives of the Hydrographic Service. The empty
spaces have not yet been resurveyed since the creation of
the digital archives.
RWS North Sea, part of Rijkswaterstaat (RWS),
which is the Dutch Directorate-General of Public
Works and Water Management, is responsible for
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INTERNATIONAL HYDROGRAPHIC REVIEW
Both authorities have developed a grid-point wise
prediction setup. As a consequence, they only analyse
vertical changes in depth. Horizontal changes,
that occur when a sand wave migrates, are not taken
into account. We propose a combination of both
methods that allows incorporating an area wide sea
floor representation into grid-point wise predictions.
We first apply a deformation analysis to detect outlying
surveys and to determine sand wave migration
parameters. Furthermore, we extend the state
space model with a local testing procedure and a
sand wave propagation model, based on the parameters
found in the deformation analysis. Results of
all three methods are demonstrated on a time series
of nine surveys in 11 years of an area with a
moving sand wave.
2. Current Prediction Methodologies
Figure 2: Current resurvey schedule, RWS/DNZ denotes
RWS North Sea.
the maintenance of shipping channels, like the Euro
Channel to the port of Rotterdam. For this channel,
a guaranteed nautical depth is defined: when the
depth in the channel becomes too shallow it has to
be dredged. To predict the moment when the channel
floor will rise above a critical depth (Wüst 2004)
introduced a trend analysis model, based on a state
space approach with Kalman filtering updating (Ka-
Iman 1960). In this method a space-time representation
of the sea floor is estimated, using local linear
growth models (West and Harrison 1997). The state
at each grid point of the modelling grid consists of
a constant depth part and a linear trend over time,
together defining a local linear growth state space
model. The states are updated when new measurements
become available. The information of the
relatively fine archive grid depth measurements is
projected on the coarser modeling grid using a Gaussian
kernel method. A probabilistic prediction of the
future sea depth is made using the estimated state,
consisting of depth values and trends, and its variance-covariance
matrix. This method produces predictions
for future depths together with a calibrated
predictive uncertainty distribution. It is calibrated for
predictions up to five years ahead.
In this section, two methods are described for
predicting future depth values, based on repeated
soundings. It should be noted that the methods as
given here, represent the situation of 2004 (Dorst
2004 and Wüst 2004). First we sketch the method
under development at the Netherlands Hydrographic
Service, then we give an overview of the method
as implemented back in 2004 at RWS North Sea.
More recent developments in the methodology of
the Hydrographic Service can be found in (Dorst et
al. 2007).
2.1 Methodology Hydrographic Service
The applied estimation and prediction method of
the Netherlands Hydrographic Service is based on a
least squares analysis where alternative representations
are statistically compared in a testing procedure
(Koch 1999, Teunissen 2000, 2001). It is assumed
that depth values and depth accuracies are
given for the nodes of a fixed grid for each survey.
The accuracies describe the combined effect of all
the measurement and processing errors. In a first
step the most appropriate static representation of
the sea floor is determined. In the second step data
from all available surveys is analyzed to determine
the dynamics of the sea floor starting from the static
representation.
2.1.1 Static sea floor representation
For approximating the sea floor, alternative representations
are considered. The initial static representation
is a horizontal plane, parameterized by one over-
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all depth parameter, d. That is, the expected depth
E{ d p } at any point p of the sea floor equals d:
(1) E{d p } = d , for any point p
One hypothetical extension is a sloping plane, using
two additional parameters for the slopes in x-
and y-direction. A second hypothetical extension is
a plane superimposed by a sine wave representing
a one-dimensional sand wave. That is a sand wave
such that consecutive sand wave crests are straight
and parallel. As a consequence, the orientation of
a sand wave field can be described by one azimuth
angle, East of North, perpendicular to the crest
lines. The grid spacing has to be dense enough
with respect to the wavelength of the sand waves,
to prevent aliasing effects. Typical values are 50 to
100m. Wavelength is considered to be a constant
here, both spatially and temporally.
The representation of the sea floor as a horizontal
plane with a superimposed one-dimensional sand
wave expresses the expected depth E{ d P } at point
p with horizontal coordinates (x P , y P ) as
with d the mean depth of the area and L the sand
In this way, the depth d at position p depends linearly
on the estimation parameters (d, u, v). By the
of fit of the different representations, the number of
model parameters and the quality of and possible
correlation between the observations are taken into
account. Subsequently, the model is extended with
the most relevant extension. This procedure continues
until none of the available alternatives significantly
improves the fit of the model to the observations
anymore.
2.1.2 Area dynamics
The dynamic behaviour of the parameters is used to
deduce depth changes, sand wave growth and sand
wave migration, in a procedure similar to (De Heus
et al., 1994). In this case, the initial scenario is a
sea floor that is static throughout all surveys. As alternative
hypotheses a linear trend and single outlying
surveys are considered. In this context, a survey
is considered outlying if its describing parameters
do not match with the parameters of the other surveys,
either due to a change in sea floor, or because
of a deviation in the measurement process. More
complex, non-linear behaviour of consecutive surveys
is represented as a combination of several outliers,
possibly including a linear trend. The method
starts with the static sea floor representation as determined
in the static analysis step. Subsequently,
a statistical hypothesis is set up for each extension,
and a corresponding test statistic and critical value
are calculated, again by incorporating an appropriate
VC-matrix. Each test statistic is divided by its
critical value, to obtain a test quotient. The critical
value depends on the user defined level of significance,
which is the probability that an extension is
accepted by the testing procedure, while in fact it
should be rejected. The extension with the highest
test quotient is the most significant extension and
is therefore added to the initial static representation
for representing the dynamic behaviour. Note that
this procedure is similar to the one described at the
end of paragraph 2.1.1.
Selection of the most likely sea floor model is done
by a testing procedure. In such a procedure, the observations
are first fit to the most simple sea floor
model. Here the simplest model is the representation
of the sea floor as a horizontal plane, see
Equation (1). If the observations fit badly, alternative
models are considered that are less simple, like for
example the representation of the sea floor as a horizontal
plane with a superimposed one-dimensional
sand wave, Equation (2). In evaluating the degree
The dynamic model of a trend in the sand wave is
constructed by extending an initial sea floor model
for one epoch, as the one in Equation (2), with a
time, t, dependent term
Here t 0 indicates the time of the first epoch. Fitting
the observations of all available epochs into this
model will result in wave propagation parameters
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2.1.3 Outlier removal
Similar testing procedures can be applied to determine
relatively small outliers that may remain after
a first data cleaning procedure. The well-known w-
test compares the fit of all observations to the fit
where one observation is discarded. If the latter fit
is significantly better, the discarded observation is
considered an outlier, (Teunissen, 2000). Similar to
the w-test for single soundings, also a single survey
could be qualified as an outlying survey by the testing
procedure, for instance if the depth is systematically
overestimated, due to e.g. a systematic error in
the measurements.
2.1.4 Point dynamics
In a similar way as for a complete area, dynamics
can be estimated from the time series of depth values
and depth accuracies as available at a single
grid node. For a single node position, typically about
four to ten depth values are available through time.
Therefore, it is only feasible to represent the depth
dynamics at a point location with only a few parameters.
Particularly, the initial scenario of a static
depth is tested against the alternative hypothesis of
a linearly increasing or decreasing depth. It is also
considered if one or more nodal depths should be
disregarded because their depth value is not in line
with the other depths, using the outlier hypothesis.
Adaptation of a certain representation by the testing
procedure does not only result in a statistical optimal
estimation of the parameter values describing
the grid point behaviour, but also in their variances,
describing the adequacy of the representation. Note
that in all cases described here the Hydrographic
Service is processing in batch mode: the unknown
parameters are estimated from all observations together
and if a new survey becomes available, the
computations are redone on all observations again.
RWS North Sea has developed a trend prediction
model for monitoring the sea floor depth (Wüst
2004). There are two main differences with the previous
approach. First a whole area, containing many
grid points, is processed at once. This enables a
joint accuracy calibration that takes correlation between
nearby grid-points into account. Second, new
observations are not processed in batch mode but
recursively: the updated state estimate is a direct
combination of the existing, previous state estimates
and the newly available observations. Elsewhere,
(Knaapen et al. 2006) it is demonstrated
how, with a small adaptation, this trend prediction
model can also be used to monitor regeneration of
sand waves after dredging.
2.2.1 Kalman filtering
Recursive state estimation is generally known as
Kalman filtering, after (Kalman, 1960), who initially
described the methodology. A typical example of recursive
estimation is navigation, where an updated
estimate of the actual position has to be calculated
continuously and instantly from for example the
available GPS observations. In bathymetric processing
a Kalman filter approach has been proposed for
the automatic updating of depth estimates when
passing a certain position several times within one
survey (Calder and Mayer 2001). A bit of an inverse
application is to track the position of an AUV or submarine
by continuously comparing its soundings
with a given georeferenced multibeam chart (Jalving
et al. 2004).
The state-space model of RWS North Sea maintains
a state vector xk through time, representing the
state of the linear growth models which represent
the dynamics of the sea floor at a suitable grid covering
the area of interest. The state vector contains
the local depth and depth trend for each grid point,
d, i=l..J, that is
Predictions for future depth values are obtained
by extrapolating the trends at each grid node into
the future. The variance of a trend-based prediction
will increase with the increase in time between the
moment of prediction and the moment of the last
sounding.
2.2 Methodology RWS North Sea
In order to schedule survey and dredging activities,
Parallel to the state vector its variance-covariance
(VC) matrix Q xk is estimated, containing the variances
of the state vector on the diagonal, while the offdiagonal
elements contain the possible covariances
between the state parameters. The state vector can
be update in the following two different ways (Teunissen
2001).
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2.2.2 Time update
A time update is performed when the state of the
sea floor is needed at a moment when no surveys
are available, e.g. in case of a prediction. The local
depths and depth trends are extrapolated to the moment
of the time update. In matrix notation this is
represented as
2.2.3 Measurement update
A measurement update is performed whenever a
new survey becomes available at time t. In a Kalman
filter update step, the state parameters are adapted
to the new survey data. The effect of the new observations
on the state parameter depends on the accuracy
of the previous state, on the accuracy of the
new observations and on the spatial relations between
the observations and the state parameters.
The exact formulas for a measurement update can
be found in (Teunissen 2001) or in (Zarchan and
Musoff 2000).
The parameter σ denotes a suitable discount fac-
diction results on real observations. This discount
method is adapted from (West and Harrison 1997).
The variance of the depth values, available at a
5m grid, consists of two parts. One part is determined
from the variability between the soundings
that contribute to a grid cell value; the other part
is a fixed measurement noise component that after
calibration is set at 0.23 m 2 . The measurement
noise component takes both the accuracy of the
echo sounder and short-scale variability due to the
presence of small unpredictable morphological variation
into account. The Kalman filter is calibrated
to maintain depths and depth trends at a grid with
a grid spacing of 17.5m. The functional relation between
the depths at the model grid points and the
5 m observational grid is given by a weight matrix
whose weights are determined according to the distance
between the archive points and the model grid
Hydrographic Service
RWS North Sea
Stored archive values
Survey archive quality
description
Main goal of time series
analysis
Method
Data assimilation
Regridding
Point-wise dynamics
Area-wise dynamics
Prediction
Shallowest depth of SBES data
in 5m grid cells
A priori: derived from the
specifications of the sensors
Insight in sea floor dynamics
Deformation analysis
Batch
Archive data are regridded to coarse
grid (e.g. 60m) by Kriging
Static, outliers, linear trend
Static, outliers, linear trend, possibly
including a 1D sand wave
Point-wise, based on detected
dynamics
Mean depth of MBES data in 5m grid
cells.
A posteriori: derived from the
measurements
Prediction of crossing time of nautical
limits
State space model with Kalman
filtering
Recursive
Archive data are regridded to coarse
grid (e.g. 17.5m) by Gaussian
interpolation Kernel
Linear trend
Not directly incorporated, but indirectly,
via the stochastic model.
Point-wise, based on the local linear
trends
Table 1: Overview methods RWS North Sea and Netherlands Hydrographic Service.
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points by means of a Gaussian kernel function, (Lee
et al. 2002). For the kernel width (standard deviation)
also a value of 17.5m is used.
2.3 Comparison of both existing methods
In Table 1 an overview of both methods is given.
MBES denotes multibeam echo sounding, SBES
indicates single beam. The top part of the table
shows that not only the methods itself differ, but
that also differences exist in the storage of survey
data. Both authorities process the raw soundings to
a 5m grid. The Hydrographic Service uses the shallowest
SBES-sounding per 5m cell for its analyses,
while RWS North Sea is using the cell average of the
MBES values. The latter approach is less sensitive
to uncorrected measurement errors. As a consequence,
both authorities would end up with different
predictions, even when using the same prediction
methodology and using the same soundings.
The main difference between both approaches logically
results from their different goals: The Hydrographic
Service decides between different dynamics
using hypothesis testing methodology and determines
predictions at a certain grid node based only
on the available archive data at that particular node.
RWS North Sea produces calibrated predictions using
a space-time modelling approach.
A relevant practical difference is that the Hydrographic
Service processes all historical data including the
latest survey in batch mode while RWS North Sea
applies a recursive method. As a consequence RWS
North Sea only needs to process the new observations
when updating the model, provided that the
model state as estimated on the basis of the previous
observations has been stored for later use.
by a 1D sand wave, but the principle also works for
other types of spatial dynamics.
3.1 Synthetic sand wave migration
Consider a simulated propagating sand wave with 1
m amplitude and 300 m wave length. Every year, the
crest is shifted 10 m to the right. Figure 3 shows
the sand wave positions in four consecutive years.
In Figure 4 the analysis results of a deformation
analysis per grid-point are given. Points on the left
of the crest become deeper, while points on the right
become shallower. This simulation clearly demonstrates
a shortcoming of predictions based on local
linear trends: the prediction quality decreases with
time, as these points in reality do not show a linear
trend in time but a harmonic trend. Such predictions
can be improved by incorporating a spatial, area
based sand wave model into the grid-point wise predictions,
as this allows for modeling the sand wave
propagation. The essential extension is that in this
case a global model is used for predicting local dynamics.
Figure 3: Profile of a simulated migrating sand wave.
3. The proposed new methodology
The sea floor prediction methods developed by RWS
North Sea and the Hydrographic Service are both restricted
to modelling sea floor dynamics in a vertical
and local sense. So, horizontal aspects of sea floor
dynamics, due to e.g. sand wave migration, are not
taken into account, e.g. due to sand wave migration.
This is demonstrated in a synthetic example in Section
3.1. In Section 3.2 a new method is introduced
that allows incorporating spatial dynamics found for
a complete area into node predictions. The method
is described and demonstrated for an area covered
Figure 4: Depth predictions.
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3.2 Local-global predictions
All ingredients needed for local depth predictions
while incorporating a spatial dynamic model have
been introduced in Section 2. Here we will show how
to incorporate the dynamics of the area in the local
prediction of the Kalman filter and how to build in a
procedure to detect and eliminate outliers.
3.2.1. Using sand-wave parameters for local depth
predictions
Let us assume that the test procedure of the Hydrographic
Service shows that the area at hand is
best described by a horizontal plane with a superimposed
1D sand wave. We assume that the sand
wave orientation and length are given. The observations
from all available, say, K surveys are used to
describe the dynamic behaviour of the area through
time by means of the procedure of Section 2.1. This
analysis results in parameters Δu/Δt and Δv/Δt describing
a constant sand wave propagation velocity
and a constant change in sand wave amplitude as
indicated in Section 2.1.
In order to express this above dynamic sand wave
representation in a state space format, the state
vector x K
as introduced in Section 2.2 has to be
extended by the parameter values u, v, describing
the global sand wave itself, and by Δu/Δt and Δv/Δt,
describing the change of the sand wave over time:
3.2.2 Local-global prediction algorithm
The steps leading to a grid-point wise prediction, as
explained in some detail above, are summarized in
the following algorithm.
Input: Data grid available in K surveys. At every grid
point a depth value and a depth variance is available.
1. Determine sand wave length and sand wave direction.
2. Determine sand wave velocity, sand wave amplitude
and outlying surveys from a deformation
analysis of all K surveys. Remove outlying surveys
if found.
3. Run a state space model on the remaining data.
At every Kalman filter update step, grid-point
wise outliers are identified and eliminated.
4. Perform state space evolution step for a prediction
at an arbitrary future moment.
Output: grid-point wise predictions.
4. Prediction Algorythm Results
Here results of the methods of the Hydrographic
Service and RWS North Sea on a real data set representing
a regular sand wave are compared to results
obtained by our new, combined method. All three
methods were implemented in Matlab.
The transition from state k to state k+1, with A' =
(A k
+ ΔA)/ A k
, for the sand wave parameters only is
given by:
This transition should be added to the transition
matrix , compare Equation (5). For initializing the
extended Kalman filter, the sand wave parameters
values are taken from the test procedure while the
change in sand wave is initially set to zero. Similarly,
the initial variances of the sand wave parameters
are as obtained from the testing procedure. Without
giving the details here we mention that an outlier
removal procedure as sketched in Section 2.1.3 can
easily be incorporated in the Kalman filter as well,
(Teunissen and Salzmann, 1989).
Figure 5: Left: position of area NA. Right: depths of area
NA in 1991 in meters. Coordinates are in UTM31-WGS84.
4.1 Data set NA
Area NA is located in the Euro channel at about 35
km off the Dutch coast, see Figure 5, left. This area
of 330 x 330m has been monitored by Multi Beam
Echo Sounding in all years between 1991 and 2001,
except for 1992 and 1998. The data are available
on a 5m grid; therefore every survey is represented
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by 4,356 grid points. Each grid point value is the
average of the soundings in the corresponding grid
cell. RWS North Sea is responsible for guaranteeing
the depth in the Euro channel. The depths in 1991
are shown in Figure 5, right. In the middle of the
figure the crest of a sand wave is visible. This sand
wave has a wavelength of about 225m and its average
orientation is 47 degrees east of North.
4.2 Deformation analysis results
Least squares adjustment of all the available surveys
to the dynamic sand wave model as described
in Section 2 gives a sand wave amplitude of about
1.4 m and a sand wave propagation velocity of 1.6
m/year. In Figure 6, nodal predictions as derived
from the point-wise deformation analysis method
are given on a 33m grid, showing upward and downward
trends on opposing sides of the sand wave
crests. These trends are caused by the sand wave
migration.
4.3 Local Kalman filter results.
In Figure 7 predictive moments of crossing of the
dredging depth are shown as determined by the
local Kalman filter approach using all surveys.
These overstep moments are based on the mean
values (50 % probability). The Kalman predictions
are in agreement with the deformation analysis results:
crossings of the critical depth are predicted to
occur at the North-East side of the two crests. As the
sand wave migrates to the right, these points were
moving upward in most surveys. The continuously
decreasing depth at these positions results in an
overstep warning. In reality one might expect that,
after passage of the crest, the sea floor at these
critical points is becoming deeper again.
Figure 6: Results of the deformation analysis per node,
colour intensity corresponds to size of the trends found.
Additionaly, values for the maximum trend and the outlier
are given. Coordinates are in UTM31-WGS84.
Figure 7: Predicted moments of crossing of the dredging
depth as determined by the state-space approach. Coordinates
are in UTM31-WGS84.
4.4 Results local-global algorithm
In Figure 9, the actual data in 2001 and profiles
from the sand wave representation as obtained from
running the local-global Kalman filter for 10 years
are shown. The location of the profiles is indicated
in Figure 5, right. In contrast to similar profiles for
1991 (not shown) at the initialization of the Kalman
filter, the profiles fit well. It is also clear that the
shape of the sand wave is not a sine in reality.
Figure 8 gives predictions one year ahead for the
individual grid points based on the new local-global
algorithm. That is, the differences between the observations
of the last survey and the predictions for
one year ahead are displayed. Clearly, the motion
of the sand wave in North-East direction is visualized
by the pointed triangles, indicating upwards or
downwards movements of at least 5cm. A validation
of the prediction results is obtained by comparing
predictions for e.g. 2001 to the actual grid wise observations.
The mean absolute difference between
prediction and observation is +12cm, with maximum
values of +45cm and -12cm. Maximal differences
occur near the crests and are probably due to the
mismatch between the sinusoidal representation
and the actual sand wave shape. The main benefit
of the local-global approach is that it allows incorporating
obvious global dynamics in predicting depth
changes at individual grid points. As a consequence,
more reliable predictions can be obtained in case of
regular, horizontal sea floor dynamics.
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INTERNATIONAL HYDROGRAPHIC REVIEW
Figure 8: One year ahead predictions as derived by the
local-global algorithm.
5. Discussion and Conclusions
We propose a method to incorporate an area wide
morphological sea floor model in a state space Kalman
filter model for the purpose of grid-point wise
change predictions. In this particular case a propagating
sinusoidal sand wave was modeled. The test
results indicate that, using the new approach, potentially
more reliable and therefore more cost-effective
predictions are obtained for areas with 1D moving
sand waves. A remaining problem is that sand
waves are in reality not sinusoidal. Because the proposed
method includes a stochastic component, a
not-optimal fit will automatically result in predictions
with a larger variance. Still it is recommended to
consider alternative low parameter representations
of sand waves.
Dynamic areas with an irregular morphology can be
automatically reported by the deformation analysis
component as fitting badly to any tested dynamical
model. In such a case the area could be segmented
in more regular sub-areas. Suspect areas should
not only be resurveyed more often but also analyzed
at higher resolution than relatively safe areas. In the
methods described here, it is silently assumed that
changes occur regularly. It is not clear to what extent
this assumption exactly holds.
A large inventory of bathymetric data of the Netherlands
Continental Shelf will increase the insight of
the applicability of the discussed methods, the kind
Figure 9: Two profiles of the modeled (blue) and real (red)
sand waves. The location of the profiles is indicated in
Figure 5.
of sea-floor representations needed and the validity
of the assumptions made. For this purpose an overall
systematic data analysis procedure could be performed
on the available soundings in the bathymetric
archives of e.g. the Hydrographic Service or RWS
North Sea for those specific areas where several
surveys are available.
Acknowledgements
The authors thank Simon Bicknese, Jelle de Plaa
and the anonymous reviewer for their valuable comments.
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Biographies
Roderik Lindenbergh studied Mathematics at the
University of Amsterdam. He obtained a PhD in
Mathematics from the University of Utrecht on his
work on Voronoi diagrams. After his PhD he joined
Delft University of Technology to work in the section
of Mathematical Geodesy and Positioning. Current
research interests include deformation analysis,
spatio-temporal interpolation and quality control of
large spatial data sets. Roderik works on the integration
of GPS and MERIS water vapor data and is
involved in the analysis of ICES at full waveform laser
altimetry data and in the quality aspects of terrestrial
laser scanning.
Leendert Dorst finished his MSc in Geodetic Engineering
at the Delft University of Technology in
1999. He has been employed at the Netherlands
Hydrographic Service since. His tasks include consultations
on hydrographic surveying, maritime positioning,
coordinate systems, and technical aspects
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INTERNATIONAL HYDROGRAPHIC REVIEW
of the law of the sea. He participates in the IHO
S44 working group on Standards for Hydrographic
Surveys. Since 2004, he has been a PhD candidate
at the University of Twente, studying the analysis
of time series of bathymetric surveys using deformation
analysis, to improve the resurvey policy of the
Netherlands.
Hans Wüst studied Naval Architecture at TU Delft.
He worked at Rijkswaterstaat North Sea on probabilistic
admittance policy and on hydro-meteorological
predictions like water level, current, and swell using
neural networks. He developed an operational error
correction method for systematic periodic errors of
numerical water level predictions using Kalman filtering
and Bayesian techniques and a Kalman Filter
sand wave prediction model. Currently he is working
at the Rijkswaterstaat Centre for Transport and
Navigation as statistical consultant.
Peter Meriting studied Geodetic Engineering at the
TU Delft. He wrote his master thesis on the detection
and prediction of sea floor dynamics. In this
research he compared the methods of Rijkswaterstaat
and the Hydrographic Service to analyze time
series of echo sounder measurements. By analysing
time series of measurements, it appeared to
be possible to predict the behaviour of the sea
floor, especially when covered by sand waves. Currently
he is working as a project manager at Fugro-
Inpark, where he works on various land surveying
projects.
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Article
Encoding AIS Binary Messages in XML Format
for Providing Hydrographic-related Information
Kurt Schwehr and Lee Alexander, Center for Coastal and Ocean Mapping/Joint Hydrographic
Center, University of New Hampshire, Durham, New Hampshire (USA)
Abstract
A specification is proposed to enable hydrographic and maritime safety
agencies to encode AIS messages using Extensible Markup Language
(XML). It specifies the order, length, and type of fields contained in ITU-R.M.1371-1.
A XML schema validates the message definitions, and a XSLT style sheet produces
reference documentation in 'html' format. AIS binary messages in XML are an effective
means to communicate dynamic and real-time port/waterway information. For
example, tidal information can be continuously broadcast to maritime users and applied
to a "tide-aware" ENC. The XML format aligns with the type of data encapsulation
planned for the IHO Geospatial Standard for Digital Hydrographic Data (S-100).
Résumé
Une spécification est proposée pour permettre aux agences hydrographiques
et de sécurité maritimes d'encoder les messages AIS
(systèmes d'informations automatisés) à I'aide du langage à balises extensible (XML).
Celle-ci précise I'ordre, la longueur et le type de champs contenus dans ITU-R.M.1371-
1. Un schéma XML valide les définitions du message et une feuille de style XSLT
produit une documentation de référence au format "html". Les messages binaires AIS
en XML constituent un moyen efficace de communiquer des informations dynamiques
et en temps réel sur les ports et les voies navigables. Les informations sur les marées
peuvent, par exemple, être diffusées en continu aux utilisateurs maritimes et appliquées
à une ENC dans laquelle les marées sont prises en compte. Le format XML
s'aligne sur le type d'encapsulation des données prévu pour les normes géospatiales
de I'OHI pour les données hydrographiques numériques (S-100).
Resumen
Se propone una norma que permits a las agencias hidrográficas y de
seguridad marítima codificar mensajes AIS utilizando Extensible Markup
Languaje (XML). Especifica el orden, longitud y tipo de campos contenidos en ITU-R.M.
1371-1. Un esquema XML valida la definición de mensajes y una hoja tipo XSLT
produce documentación de referenda en formato 'html'. Los mensajes binarios AIS
en XML son un medio efectivo para comunicar información dinámica y en tiempo real
de puertos y/o vías de navegación. Por ejemplo, información sobre mareas puede ser
continuamente trasmitida a los usuarios marítimos y ser aplicada a 'aviso de marea'
de ENC. El formato XML se alinea con el tipo de datos encapsulados planificado para
el Estándar Geoespacial de Datos Hidrográficos Digitales de la OHI (S-100).
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INTERNATIONAL HYDROGRAPHIC REVIEW
Introduction
Automatic Identification System (AIS) is an autonomous
and continuous broadcast system that
exchanges maritime safety/security information
between participating vessels and shore stations.
AIS operates in the VHF maritime mobile band using
Time Division Multiple Access (TDMA) technology to
be able to meet high broadcast rates, while ensuring
reliable and robust operation.
Chapter V of the 1974 SOLAS Convention [1] requires
mandatory carriage of AIS equipment on all
vessels constructed after 1 July 2002. Implementation
for other types and sizes of SOLAS Convention
vessels was required to be completed not later than
31 December 2004.
As stated in SOLAS Chapter V, Regulation 19, section
2.4.5, [1] AIS shall:
1 provide automatically to appropriate equipped
shore stations, other ships and aircraft information,
including ship's identity, type, position, course,
speed, navigational status and other safety-related
information;
2 receive automatically such information from similarly
fitted ships;
3 monitor and track ships; and
4 exchange data with shore-based facilities.
as other information related to vessel identification,
cargo, etc.
AIS enables both ships and maritime safety administrations
(e.g., U.S. Coast Guard) to effectively track
the movement of vessels in coastal waters (See
Figure 1). In addition, AIS can contribute to safetyof-navigation
and protection of the environment by
providing additional information. This includes meteorological
and hydrographic data, carriage of dangerous
cargos, safety and security zones, status of
aids-to-navigation, and other ports/waterway safety
information. It is intended* that this information be
broadcast from shore-side AIS Base Stations to
ships that are underway at-sea or in port.
Binary Message Formats
In May 2004, the IMO issued SN/Circular 236 on
"Guidance on the Application of AIS Binary Messages"
[3] More specifically, a set of seven (7) messages
were defined with the intent that they would
undergo a 4-year trial period. The criteria for selecting
the trial messages included a demonstrated
operational need, wide cross-section of users (e.g.,
ships, VTS, pilots, port authorities), and AlS-related
messages that had already been developed in
terms of format and content.
In this regard, the IMO Performance Standards for
AIS [2] states that:
The AIS should improve the safety of navigation by assisting
in the efficient navigation of ships, protection
of the environment, and operation of Vessel Traffic
Services (VTS), by satisfying the following functional
requirements:
1 in a ship-to-ship mode for collision avoidance;
2 as a means for littoral States to obtain information
about a ship and its cargo; and
3 as a VTS tool, i. e. ship-to-shore (traffic management).
Further, AIS should be capable of providing to ships
and to competent authorities, information from the
ship, automatically and with the required accuracy
and frequency, to facilitate accurate tracking. Transmission
of the data should be with the minimum
involvement of ship's personnel and with a high
level of availability. As shown in Table 1, the contents
of an AIS message contain detailed information
regarding the location and movement as well
While it is IMO that defines the content of AIS Messages,
it is ITU-R M.1371 that specifies the technical
characteristic and the structure of the binary
AIS messages [4]. These messages have to be distinguished
from Addressed Safety-Related Messages
and Broadcast Safety Related Messages both of
which allow the exchange of format-free ASCII-text.
Binary messages are intended to reduce the need
for verbal communications, and to enhance reliable
information exchange.
To date, the content of AIS messages have been primarily
defined using text tables. Although the tables
cover a wide range of information, they are not in a
machine-readable format that facilitates rapid AIS
binary message generation. While the ITU specifies
the technical structure and IMO defined the content
(i.e., the "What"), there is a need to define "How"
to efficiently generate binary AIS messages. In
short, a XML AIS Definition Language provides the
method how to create an AIS Binary Message that
accomplishes what the tables list.
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Table 1: AIS message numbers 1-3 (Class A vessel position report) [Source: ITU-R 1371-1, Table 15a, p. 43.].
Current AIS Binary Messages
There are at least four (4) formats that have been
developed for the transmission of water level over
AIS. One example is the IMO/IALA "Metrological and
Hydrological Data" trial binary message specified by
IMO [3] and developed by IALA [5]. As shown in Table
2, this type of message contains human-readable
text. However, this method of definition can inadvertently
lead to unforeseen ambiguities. For instance,
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INTERNATIONAL HYDROGRAPHIC REVIEW
ing. The same XML descriptions
of messages can be used to generate
portions of the human-readable
specification document. Using
a XML specification allows for
rapid development of additional
AIS applications such as database
interfaces (e.g., PostGIS) [7]
and custom graphical user interfaces
(e.g., a message construction
tool for testing in the reference
implementation).
Figure 1: Vessel traffic off the New England Coast (USA) tracked using the US
Coast Guard's AIS. April 2006 traffic is plotted in black and May 2006 traffic
is in red. (Image courtesy Michael Thompson, NOAA Stellwagen Bank National
Marine Sanctuary).
the hydrographic portion contains a range of data
types that require scalings and offsets that are not
explicitly described. In turn this can cause problems
during software development. An example of a simple
ambiguous field is the "Ice" field. The field can
have values 0 through 3. It is not clear which number
applies to "yes" and "no" Additionally, if the highest
number (3) is taken to be "no data available" or
"unknown", there remains one possible value with
an undefined meaning. Another example is that the
order of Latitude and Longitude is different than in
the rest of the ITU-R M.1371-1 messages [4]. The
core messages lists Longitude first, then Latitude.
Using a more orderly and structured format such as
XML, provides an effective means to overcome these
problems.
Proposed XML Format for AIS Binary
Messages
In developing a more robust AIS
binary message, there are several
goals:
Specifications that are readable
by both humans and machines.
Allow automated testing and
validation of implementations
based on the specification.
Provide specification of the order
of fields, length of fields,
and type of fields.
Specify the scaling and offset to be applied to the
field between the application and the AIS network
layer.
Declare the units of each field when appropriate
(e.g. meters, seconds, degrees Celsius).
The specification must be independent of programming
language (e.g., can be implemented in C,
C++, Java, Python).
AIS XML Definition Language
To reduce ambiguities and ease the processes of
creating AIS software and documentation, the use of
an AIS XML Definition Language is proposed. Appendix
1 contains a reference implementation written in
Python [8] that gives a sample implementation of
code to generate encoders/decoders for an AIS binary
message. The AIS XML Definition Language is
informally known as the "AIS Binary Decoder Ring."
It is proposed that Extensible Markup Language
(XML) [6] be used to define the binary content (payload)
for maritime-based AIS binary messages. By
providing a bit-level description in XML, producers of
binary messages will be able to more clearly specify
messages to software engineers implementing communication
systems that must handle AIS messag-
The AIS XML Definition Language draws on the best
of many existing systems. The most relevant specification
is "RFC 1832 -XDR: External Data Representation"
[9] The AIS binary message XML specification
is a simplification of RFC-1832 converted to XML
with additions that fit the specific requirements of
AIS. For example, extensions include:
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INTERNATIONAL HYDROGRAPHIC REVIEW
Parameter
Message ID
Repeat Indicator
Source ID
Spare
IAI
Latitude
Longitude
Date and time
Average wind speed
Wind gust
Wind direction
Wind gust direction
Air temperature
Relative humidity
Dew point
Air pressure
Air pressure tendency
Horizontal visibility
Water level (incl. tide)
Water level trend
Surface current direction
Current speed, #2
Current direction, #2
Current measuring levevl #2
Current speed, #3
Current direction, #3
Current measuring level, #3
Significant wave height
Wave period
Wave direction
Swell height
Swell period
Swell direction
Sea state
Water temperature
Precipitation (type)
Salinity
Ice
Spare
Total Number of bits
No. of bits
6
2
30
2
16
24
25
16
7
7
9
9
11
7
10
9
2
8
9
2
9
8
9
5
8
9
5
8
6
9
8
6
9
4
10
3
9
2
6
352
Description
Identifier for Message 8; always 8
Used by the repeater to indicate how many times a message
has been repeated.
MMSI number of source station
Not used. Should be set to zero.
DAC=001; FI=11
Measuring position, 0 to +/-90 degrees, 1/1000th minute
Measuring position, 0 to +/-180 degrees, 1/1000th minute
Time of transmission, Day, hour, minute, (ddhhmm in UTC)
Average of wind speed values for the last 10 minutes
Wind gust is the maximum wind speed value reading during
the last 10 minutes, 0-120 kts, 1kt
0-359,1 degree
0-359, 1 degree
Dry bulb temp. - 60.0 to +60.0 degrees Celsius 0.1 of a degree
0-100, 1%
- 20.0 - + 50.0 degrees, 0.1 degree
800-1200 hPa, 1hPa
0 = steady, 1 = decreasing, 2 = increasing
0-25.0, 0.1 NM
Deviation from local chart datum, -10.0 to 30.0 m
0 = steady, 1 = decreasing, 2 = increasing
0 - 359 degrees, 1 degree
Current measured at a chosen level below the sea surface,
0.0 - 25.0 kts, 0.1 kt
0 - 359, 1 degree
Measuring level in m below sea surface, 0-30m, 1 m
0.0 - 25.0 knots, 0.1 knot
0 - 359 degrees, 1 degree
Measuring level in m below sea surface, 0-30 m, 1 m
0.0-25.0 m, 0.1 m
Period in seconds, 0-60 s, 1 s
0-359 degrees, 1 degree
0.0-25.0 m, 0.1 m
Period in seconds, 0 - 60 s, 1 s
0 - 359 degrees, 1 degree
According to Beaufort scale (manual input), 0 to 12, 1
-10.0 - + 50.0 degrees, 0.1 degree
According to WMO
0.0 - 50.0 0/00, 0.1 0/00
Yes/No
Occupies 2 slots
Table 2: IMO Meterology and Hydrology Message as specified in IMO SN/Circ.236, Annex 2, Application 1. Also described
in AIS, Vol 1, Part 1, Operational Issues, Ed. 1.3. IALA Guildeline No 1028, p. 131.
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INTERNATIONAL HYDROGRAPHIC REVIEW
Bit level field lengths allowing for non-byte align
data
Scaling and offsets of encoded data to increase
information density
Units
Mandatory human-readable description of each
field
AIS XML Definition Language meets all of the goals
and relies on industry standard technologies. There
are many libraries that support reading and validation
of XML documents.
XML schemas can be crafted to validate binary message
definitions. These schemas might be written
in XML Schema [10], Schematron [11], or RelaxNG
[12]. Designers can validate draft message definitions
using these schema and/or additional validation
programs.
Using an AIS XML Definition Language allows for the
packaging of test data to validate both encoding and
decoding of messages. For each message, a number
of example messages can be defined such that the
major corner cases are tested by all AIS software
vendors. Each example XML test message will contain:
ASCII encoded binary containing the bit stream represented
by "0" and " 1 " characters
The NMEA strings as they would be returned by an
AIS modem
The fields broken out with scaling removed
The reference software contains several example
XML test messages. They are encoded in a general
way such that a new format is not required for each
new message type. One practical example would be
for Maritime Domain Awareness (MDA). MDA is an initiative
within the US Department of Homeland Security
that seeks to rapidly process massive amounts of
maritime data and information [22]. XML interoperability
standards and exchange formats would allow
software to access the critical information contained
within AIS messages without having to directly comprehend
the AIS binary format.
Basic Format
The XML specification for one message is encapsulated
in a XML "message" tag. The message contains
the necessary information to serialize and
deserialize AIS message information to and from
the AIS binary message payload and the local machine
representation used within an application.
Messages are wrapped within a XML header and an
outer 'AlS-binary-message" tag. This tag can contain
multiple messages. Xlnclude [13] allows inclusion
of predefined standard structures such as time
stamps and positions, which are used in many of
the messages.
The message tag contains attributes that specify
the name, AIS message number, the Designated
Area Code (DAC), and the Functional ID (FID). The
attributes for DAC and FID are repeated inside of
the XML message definition as fields to facilitate
parsing of messages. In order to know what message
is being examined, the first bits of the message
must be decoded. The ITU 1371 specification
of messages 6 and 8 states that the message begins
with 16 bits of the application ID. The DAC is
the designated area code (e.g., 1 for IMO, 366 for
the USA). The FID indicates for which application
this data is intended. The IMO trial Met/Hydro format
message has a DAC of "1" and a FID of "11."
For AIS binary messages, AIS message number can
be "6", "8", or "6 8" depending on if the message
is addressed to a recipient or broadcast to all listeners.
Message 6 is the AIS Address Binary Message
(ABM) where as message 8 is the Broadcast
Binary Message (BBM). As the ITU AIS specification
is revised, there may be additional message numbers
that carry binary message data.
These additional message attributes provide assistance
to other applications that might want to
develop new AIS technologies that are driven by the
XML definition. For example, in Appendix 1, the designer
of the AIS XML Definition Language specification
for Met/Hydro suggests that displayed Met/Hydro
messages be titled by the UserlD (i.e., MMSI).
The reference Python code uses this to title Google
Earth popup messages.
An AIS Binary message is composed of a list of
fields. At a minimum, each field contains a name,
number of bits, and type attributes. The name is
comprised of alphanumeric and underscore characters.
Each field is required to have a human-readable
description tag. This tag will appear in the
"Description" column for all of the generated message
documentation (e.g., text documents and web
pages).
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Each field has XML tags and attributes that specify
information about the field and its representation.
Each field must have a data encoding type selected
from one of the available types listed in Table 3.
If a field requires a non-varying specific AIS binary
encoded value, it would be listed in a "required"
tag. For example, messages that can only be sent
as binary broadcast messages will have this value
be set to 8 for the "MessagelD" field. For fields that
can change, it is possible to apply scaling and offsets
to the values to pack them into smaller numbers
of bits than might be otherwise available. If
the number of bits provides a greater range than is
needed, the field can be limited using a "range". For
instance, current flow direction requires nine bits for
a range of 0-359 degrees. In terms of binary coding
this means that 29 (512) is required to achieve at
least 359 positions. Usually a particular value is selected
to represent that no information is available
from the sender for this field. All fields must have a
'units field' if units are appropriate. Units might be
degrees (as in compass direction), degrees Celsius,
meters, seconds, etc.
An example of the end result of using the AIS XML
Name
bool
uint
int
udecimal
decimal
float
double
aisstr6
ascii7
binary
Description
boolean
unsigned integer
signed integer
unsigned decimal
signed decimal
ieee floating point
ieee floating point
as defined in the
AIS specification
ASCII character
codes
binary blob
Size
1 bit
variable (1..64 bits)
variable (1..64 bits)
variable (1..64 bits)
variable (1..64 bits)
32 bits
64 bits
6 bits
7 bits
variable
Table 3: Binary message field data types
Definition Language is shown in Table 4. In terms of
content, this table contains the same IMO Meteorology
and Hydrography messages as Table 2. However,
the information used to generate Table 4 is
both human and machine-readable - and far easier
to implement. Appendix 2 provides an example of a
XML message definition converted to a webpage using
a XSLT style sheet. This webpage describes the
name, number of bits, array length, type, units and
a brief description. In turn, this matches the style of
other AIS standards specifications.
Application
AIS binary messages have a wide range of potential
applications that would benefit the hydrographic and
maritime user community. In concept, AIS can be an
effective means to digitally communicate relevant
ports/waterways information related to dynamic
and real-time information [14]. One example of a
relevant application would be the "Next Generation"
Electronic Chart whereby tidal information is continuously
broadcast to maritime users either inport
or underway and applied to "tide-aware" Electronic
Navigation Charts (ENC) capable of accommodating
x, y, z, and time [15,16].
In the USA, the challenges to accomplish these applications
are more organisational than technical.
The process requires establishing the necessary
infrastructure to take the water level information at
a particular tide station zone/area from the NOAA
Physical Oceanographic Real-Time System (PORTS)
[17] and the NOAA CO-OPS [18] systems, convert
it into an XML message, and then transmit
it via USCG AIS Base Stations to ships in the
area. To accomplish the process will require
a fair amount of cooperation between government
agencies, ECDIS manufactures, and maritime
user groups.
Where water levels are transmitted over AIS, finite
element modeling software such as TCARI
[19] can be used produce a new water surface
every six (6) minutes. Potentially, this can be
used by both commercial navigators and for
hydrographic surveys. Hydrographers would be
able to immediately integrate highly-accurate
water levels into their processing software to
produce better initial results prior to completing
the survey cruise. In the future, when electronic
chart data is based on gridded data format, each
node on the grid could contain a "z" value (i.e.,
depth) that is made "tide-aware" [20].
Research is ongoing at NOAA, USCG, and UNH as to
the best means to convert predicted, forecast, and
"nowcast" water levels into an AIS Binary Message
format. At present, existing water level messages
do not contain the required water level information
needed for finite element modeling. However, the
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Parameter
MessagelD
Repeatlndicator
UserlD
Spare
dac
fid
latitude
longitude
day
hour
min
avewind
windgust
winddir
windgustdir
airtemp
relhumid
dewpoint
airpressure
airpressuretrend
horizvis
waterlevel
waterleveltrend
surfcurspeed
surfcurdir
curspeed2
curdir2
curlevel2
curspeed3
curdir3
curlevel3
sigwaveheight
waveperiod
wavedir
swellheight
swellperiod
swelldir
seastate
watertemp
preciptype
salinity
ice
Spare
Total bits
No. of bits
6
2
30
2
10
6
24
25
5
5
6
7
7
9
9
11
7
10
9
2
8
9
2
8
9
8
9
5
8
9
5
8
6
9
8
6
9
4
10
3
9
2
6
352
Description
AIS message number. Must be 8
Indicated how many times a message has been repeated
MMSI number of transmitter broadcasting the message
Reserved for definition by a regional authority.
Designated Area Code - part 1 of the IAI
Functional Identifier- part 2 of the IAI
Location of the vessel. North South location
Location of the vessel. East West location
Day 0..31
Hour 0..23
Min
Average wind speed values for the last 10 minutes.
Wind gust is the max wind speed value reading during the last 10 minutes.
Wind direction
Wind direction for the gust.
Dry bulb temperature
Relative humidity
Dew Point
Air pressure
Air pressure trend
Horizontal visibility
Water level (incl. tide)
Water level trend
Surface current speed
Surface current direction
Level 2 current speed
Level 2 current direction
Measuring level below sea surface for level 2
Level 3 current speed
Level 3 current direction
Measuring level below sea surface for level 3
Significant wave height
Wave period
Wave direction
Swell height
Swell period
Swell direction
Sea state according to the Beaufort scale
Water temperature
According to WMO
Salinity
Yes or no for the presence of ice
Must be zero
Takes 2 slots with 72 pad bits to fill the last slot
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INTERNATIONAL HYDROGRAPHIC REVIEW
AIS XML Definition Language has potential to allow
for rapid prototyping and testing of proposed
messages to ensure the final message definition
meets maritime users requirement for critical realtime
data.
Advantages
Using XML to define Binary AIS Message formats
has a number of advantages:
- XML is both human and machine-readable, and
provides a central definition that can be transformed
into required computer code.
- Although it it is the IMO and maritime safety administrations
that will decide what should be the
content of AIS messages, the XML AIS Binary
format provides a means on how current and/or
future requirements can be met.
- XML lends itself to the development of additional
tools/processes that are open-source and freelyavailable.
- In addition to maritime/hydrograhpic applications,
XML can be used for River Information Systems
(RIS) and other inland/river applications.
Alignment with IHO S-100
The XML formats align quite well with what IHO is
planning to do in terms developing a better means
of data encapsulation based on the new IHO Geospatial
Standard for Digital Hydrographic Data (S-
100) [21]. S-100 is the standard intended to be
used for the exchange of digital hydrographic data
between hydrographic offices, and for the distribution
of hydrographic data to manufacturers, mariners
and other data users (e.g., environmental management
organizations). It was developed so that
the transfer of all forms of hydrographic data would
take place in a consistent and uniform manner. To
date, S-57 Edition 3.0/3.1 has been used almost
exclusively for encoding Electronic Navigational
Charts (ENCs) for use in Electronic Chart Display
and Information Systems (ECDIS). However, there
are changing requirements, customers and technology
for hydrographic data and as S-57 is intended
to support all types of hydrographic data, not solely
ENCs, it needs to be expanded in order to accommodate
these new requirements.
Biographies of the Authors
Dr. Kurt Schwehr is a Research Assistant Professor
at the Center for Coastal and Ocean Mapping
(CCOM) at the University of New Hampshire. In addition
to AIS broadcast applications, he works on a
range of projects including developing Chart-of-the-
Future technologies, visualization techniques for underwater
and space applications, and understanding
marine sedimentary processes. He received his PhD
from Scripps Institution of Oceanography in marine
geology/geophysics, and received a B.S. from Stanford
University. Before joining CCOM he worked at
the Jet Propulsion Lab, NASA Ames, the Field Robotics
Center at Carnegie Mellon, and the US Geological
Survey Menlo Park. He is a team member for the
NASA Phoenix Mars Lander.
Dr. Lee Alexander is a Research Associate Professor
at the Center for Coastal and Ocean Mapping
at the University of New Hampshire, and an Adjunct
Professor at the University of New Brunswick. He is
also a Principle Investigator at the ECDIS Laboratory,
University of Southern Mississippi. Previously a Research
Scientist with the US Coast Guard and a Visiting
Scientist to the Canadian Hydrographic Service,
he serves on a number of international committees
and working groups dealing with electronic charting
and shipboard navigation system standards. He has
published over 100 papers and reports on electronic
chart-related technologies, and is a co-author of a
textbook on Electronic Charting.
References
1 International Convention for the Safety of Life at
Sea; Consolidated text of 1974 SOLAS Convention,
the 1978 SOLAS Protocol, and the 1981 and
1983 SOLAS Amendments, International Maritime
Organization, 1 July 1986, London,.
Revised SOLAS Chapter V, IMO Resolution
MSC.99(73), International Maritime Organization,
5 December 2000, London.
2 Recommendation on Performance Standards for
an Universal Shipborne Automatic Identification
System (AIS), IMO Resolution MSC.74(69). Annex
3, International Maritime Organization, 12 May
1998, London.
3 Guidance on the Application of AIS Binary Messages,
IMO SN/Circ. 236, 28 May 2004, Interna-
45

INTERNATIONAL HYDROGRAPHIC REVIEW
tional Maritime Organization, London.
4 Technical characteristics for a universal shipborne
automatic identification system using time division
multiple access in the VHF-maritime mobile band.,
Recommendation ITUR M.1371-1:98, 2001. International
Telecommunication Union.
5 Automatic Identification System (AIS), Volume 1,
Part 1, Operational Issues. IALA Guideline No.
1028, Edition 1.3, December 2004, International
Association of Marine Aids to Navigation and
Lighthouse Authorities, St. Germaine en Laye,
France.
6 Extensible Markup Language (XML) 1.0 (Fourth
Edition) [http://www.w3.org/TR/2006/REC-xml-
20060816]
7 PostGIS - Open source spatial database technology
[http://postgis.refractions.net]
8 Python - Dynamic object-oritented programing language
[http://www.python.org/]
9 R. Srinivasan. RFC 1832 - XDR: External Data
Representation Standard. Internet Request For
Comments, 1832 pages, 1995, [http://www.
faqs.org/rfcs/rfcl832.html]
10 W3C. XML Schema Part 1: Structures, 2 nd Edition
[http://www.w3.org/TR/2004/REC-xmlschema-1-20041028/]
11 Schematron [http://standards.iso.org/ittf/PubliclyAvailableStandards/c037605_IS0_IEC_
19757-2_2003(E).zip]
[http://isotc.iso.org/livelink/livelink/
fetch/2000/2489/lttf_Home/PubliclyAvailable-
Standards.htm]
12 RelaxNG
[http://isotc.iso.org/livelink/livelink/fetch/
2000/2489/lttf_Home/PubliclyAvailable
Standards.htm]
[http://standards.iso.org/ittf/Publicly
AvailableStandards/c037605_IS0_IEC_
19757-2_2003(E).zip]
13 Xlnclude [http://www.w3.org/TR/2006/
REC-xinclude-20061115/ ]
14 Pillich, Bohdan. 2003. Towards the Next Generation
ENC. Proceedings: US Hydrographic Conference
2003, Biloxi, MS
15 Brennan, R.T., C. Ware, L. Alexander, A. Armstrong,
L. Mayer, L. Huff, B. Calder, S. Smith,
M. Plumlee, R. Arsenault, and G. Glang. 2003.
The Electronic Chart of the Future: The Hampton
Roads Demonstration Project, Proceedings:
U.S. Hydro 2003.
16 Pillich, Bodan and Friedhelm Moggert. 2005.
Introducing Bathymetric ENCs on the Example
of the Port of Atlantis. Proceedings: US Hydro
2005. Conference CD.
17 NOAA, CO-OPS, 2003, Physical Oceanographic
Real Time System (Ports), [http://www.co-ops.
nos.noaa.gov/cbports/cbports.html]
18 NOAA, Chesapeake Bay Operational Forecast
System (CBOFS), [http://www.co-ops.nos.noaa.
gov/CBOFS/cbofs.shtml].
19 Brennan, Richard. T. 2005. Design of an Uncertainty
Model for the Tidal Constituent and
Residual Interpolation (TCARI) Method of Water
Level Correction. MS Thesis. University of New
Hampshire, 78pp.
20 Alexander, L. and R.T. Brennan. 2003. The Next
Generation ENC: Dealing with X, Y, Z and Time
Dimensions. Proceedings: 2 nd International
ECDIS Conference, 7-9 October 2003, Singapore.
21 Alexander, L., M. Brown, B. Greenslade, and A.
Pharaoh. 2007. Development of I HO S-100: The
New IHO Geospatial Standard for Hydrographic
Data, International Hydrographic Review, Vol.7,
No. 1, April 2007.
22 Tollefson, Eric. 2006. Strategic MDA: Applying
Fusion Technologies to Maritime Domain Awareness.
US Coast Guard Proceedings, Fall 2006,
p. 44-46. [www.uscg.mil/proceedings]
46

INTERNATIONAL HYDROGRAPHIC REVIEW
Appendix 1
Generic representation of position on the WGS84
sphereoid. Smaller number of bits than standard position.
Lat/Lon reversed.
North South location
9K/unavailable>
degrees
60000
4
37.42446
East West location
181
degrees
60000
4
-122.16328
IMO meteorological and hydroglogical data. Specified
in SN/Circ.236 Annex 2. Also defined in IALA Guidelines on AIS,
Vol 1, Part 1, Ed. 1.3. Guideline No 1028.
AII unavailable values are defined to be the highest
possible number in the next following
352
AIS message number. Must be 8
8
47

INTERNATIONAL HYDROGRAPHIC REVIEW
48
lndicated how many times a message has been repeated
0
default
do not repeat any more
l
MMSI number of transmitter broadcasting the message
This might be from a basestation or AtoN.
The transmitter might not be at the same location as the Met/Hydro station
1193046
Reserved for definition by a regional authority.
0
Designated Area Code - part 1 of the IAI
l
Functional Identifier- part 2 of the IAI
ll
Location of the vessel.
Day 0..31
days
3
Hour 0..23
3K/unavailable>
hours
2K/testvalue>

INTERNATIONAL HYDROGRAPHIC REVIEW
Min
63
minutes
58
description>Average wind speed values for the last 10 minutes.
knots
127
23
Wind gust is the max wind speed value reading during the last 10 minutes.
knots
127
35
Wind direction
degrees
511
329
Wind direction for the gust.
degrees
511
293
Dry bulb temperature
degrees Celsius
102.3
10
l
-40.K/testvalue>
Relative humidity
49

INTERNATIONAL HYDROGRAPHIC REVIEW
percent
127
99
50
Dew Point
FIX: should this be a udecimal
degrees Celsius
51.K/unavailable>
10
l
-19.2
Air pressure
hPa
131K/unavailable>
K/scale>
800
0
1150
Air pressure trend
steady
decreasing
increasing
unavailable
3
2
Horizontal visibility
nm
25.5
10
K/decimalplaces>
11.9
Water level (incl. tide)

INTERNATIONAL HYDROGRAPHIC REVIEW
m
41.1
10
1
-8.9
Water level trend
steady
decreasing
increasing
unavailable
3
0
Surface current speed
knots
25.5
10
1
22.3
Surface current direction
degrees
511
321
Level 2 current speed
knots
25.5
10
1
12.7
Level 2 current direction
degrees
51

INTERNATIONAL HYDROGRAPHIC REVIEW
511
122
Measuring level below sea surface for level 2
31
m
29
Level 3 current speed
knots
25.5
10
1
19.2
Level 3 current direction
degrees
511
93
Measuring level below sea surface for level 3
31
m
28
52
Significant wave height
m
25.5
10
1
22.8
Wave period
FIX: How does to fit to the power spectrum
sec
63

INTERNATIONAL HYDROGRAPHIC REVIEW
2
Wave direction
FIX: please define this better
degrees
511
187
Swell height
m
25.5
10
1
0.2
Sweii period
FIX: How does to fit to the power spectrum
sec
63
59
Swell direction
FIX: please define this better
degrees
511
1
Sea state according to the Beaufort scale
Taken from http://en.wikipedia.org/wiki/Beaufort_scaleVnote>
Beaufort scale
Calm
Light air
Light breeze
Gentle breeze
Moderate breeze
Fresh breeze
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INTERNATIONAL HYDROGRAPHIC REVIEW
54
Strong breeze
Near gale
Gale
Strong gale
Storm
Violent storm
Hurricane
Reserved
Reserved
unavailable
15
12
Watertemperature
degrees Celsius
92.3
10
-10
1
48.8
According to WMO
FIX: need a reference to the document describing this!
WMO scale index
unavailable
7
2
Salinity
FIX: by what standard Measured how
0/00
92.3
10
1
0.9
Yes or no for the presence of ice
FIX: what types of ice constitute a yes

INTERNATIONAL HYDROGRAPHIC REVIEW
Not sure. Maybe no ice
Not sure. Maybe yes ice
Not sure. Maybe not allowed
Unknown
l
Must be zero
0

INTERNATIONAL HYDROGRAPHIC REVIEW
Appendix 2
Met/Hydro Binary AIS Message rendered by XSLT style sheet from the XML definition.
Name
Bits
Array
Length
Type
Units
Description
MessagelD
6
uint
AIS message number. Must be 8
Repeatlndicator
2
uint
Indicated how many times a message
has been repeated
0: default
3: do not repeat any more
UserlD
30
uint
MMSI number of transmitter broadcasting
the message
Spare
2
uint
Reserved for definition by a regional
authority.
dac
10
uint
Designated Area Code - part 1 of
the IAI
fid
6
uint
Functional Identifier - part 2 of the
IAI
latitude
24
decimal
degrees
Location of the vessel. North South
location
longitude
25
decimal
degrees
Location of the vessel. East West
location
day
5
uint
days
Day 0..31
hour
5
uint
hours
Hour 0..23
min
6
uint
minutes
Min
avewind
7
uint
knots
Average wind speed values for the
last 10 minutes.
windgust
7
uint
knots
Wind gust is the max wind speed
value reading during the last 10
minutes.
winddir
9
uint
degrees
Wind direction
windgustdir
9
uint
degrees
Wind direction for the gust.
airtemp
11
decimal
degrees Celsius
Dry bulb temperature
relhumid
7
uint
percent
Relative humidity
dewpoint
10
decimal
degrees Celsius
Dew Point
airpressure
9
udecimal
hPa
Air pressure
airpressuretrend
2
uint
Air pressure trend
0: steady
1: decreasing
2: increasing
3: unavailable
horizvis
8
udecimal
nm
Horizontal visibility
waterlevel
9
decimal
m
Water level (incl. tide)
waterleveltrend
2
uint
Water level trend
0: steady
1: decreasing
2: increasing
3: unavailable
surfcurspeed
8
udecimal
knots
Surface current speed
56

INTERNATIONAL HYDROGRAPHIC REVIEW
Name
surfcurdir
curspeed2
curdir2
curlevel2
Bits
9
8
9
5
Array
Length
Type
uint
udecimal
uint
uint
Units
degrees
knots
degrees
m
Description
Level 2 current speed
Level 2 current direction
Measuring level below sea surface
for level 2
curspeed3
curdir3
curlevel3
8
9
5
udecimal
uint
uint
knots
degrees
m
Level 3 current speed
Level 3 current direction
Measuring level below sea surface
for level 3
sigwaveheight
waveperiod
wavedir
swellheight
swellperiod
swelldir
seastate
watertemp
preciptype
salinity
ice
8
6
9
8
6
9
4
10
3
9
2
udecimal
uint
uint
udecimal
uint
uint
uint
udecimal
uint
decimal
uint
m
sec
degrees
m
sec
degrees
Beaufort scale
degrees Celsius
WMO scale
index
0/00
Significan wave height
Wave period
Wave direction
Swell height
Swell period
Swell direction
Sea state according to the Beaufort
scale
0: Calm
1: Light air
2: Light breeze
3: Gentle breeze
4: Moderate breeze
5: Fresh breeze
6: Strong breeze
7: Near gale
8: Gale
9: Strong gale
10: Storm
11: Violent storm
12: Hurricane
15: unavailable
Water temperature
According to WMO
7: unavailable
Salinity
Yes or no for the presence of ice
0: Not sure. Maybe no ice
1: Not sure. Maybe yes ice
2: Not sure. Maybe not allowed
3: Unknown
Spare
6
uint
Must be zero
57

INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Article
GPS Techniques in Tidal Modelling
Dave Mann, Survey Support Manager, Gardline Geosurvey Ltd (UK)
Abstract
With advances in processing techniques, and the associated improvement
in positioning accuracies, it is apparent that deriving estimates
of water level from GPS height observations is becoming accepted as a viable
alternative to traditional tidal solutions. This paper outlines GPS techniques providing
high-accuracy solutions that are of potential use in tidal modelling, describes
a trial of alternative GPS systems, and presents data examples from actual survey
projects. Within the discussion the concept of accuracy versus precision is defined,
with special emphasis on the problem of vertical datums.
Résumé
Avec la progression des techniques de traitement et l'mélioration associée
dans les exactitudes du positionnement, il est évident que les
estimations de niveau d'eau dérivées des observations de hauteur du GPS sont progressivement
acceptées en tant qu'alternative viable aux traditionnelles solutions de
marée. Cet article decrit les techniques GPS qui fournissent des solutions de grande
exactitude susceptibles d'être utilisées dans la modélisation des données, décrit un
essai de systèmes GPS alternatifs, et présente des exemples de données à partir
de projets hydrographiques concrets. Dans le cadre de la discussion, les concepts
d'exactitude et de précision sont définis I'un par rapport à I'autre, et I'accent est
placé en particulier sur le problème des systèmes de référence verticale.
Resumen
Con el avance de las tecnicas de procesamiento y el mejoramiento
asociado en la exactitud en posicionamiento, es aparente que el
derivar estimaciones del nivel del agua basado en observaciones de las altura
con GPS esta siendo aceptado como una alternativa viable para las soluciones de
mareas tradicionales. Este artículo se refiere a las técnicas de GPS que proporcionan
soluciones de alta exactitud que son de uso potencial en el modelación de la
marea, describe una prueba de sistemas GPS alternativos, y presenta ejemplos
de datos de un proyecto de levantamiento real. Dentro de la discusion se define el
concepto de exactitud versus precisión, con especial énfasis en el problema de los
datum verticales.
59

INTERNATIONAL HYDROGRAPHIC REVIEW
Introduction
Hydrography is the science that deals with the
measurement of the physical properties of bodies
of water and their littoral land areas. Within this
science, special emphasis is usually placed on elements
that affect safe navigation, and hydrographic
surveys are conducted to collect the source data
required for the compilation of nautical charts and
associated publications.
The measurement of water depth, as with all survey
observations, is subject to measurement errors. In
planning a hydrographic survey, an error budget will
be calculated in order to quantify and assess the error
components. The purpose of the survey will drive
the accuracy requirements, and the error budget will
be used to determine whether the proposed solution
will meet the specifications. One of the largest
components of the hydrographic error budget is the
tidal uncertainty.
This paper outlines a traditional approach to tidal
reduction, describes the typical accuracies required,
and outlines some of the difficulties of the approach.
tions where a survey is conducted in the immediate
vicinity of the instrument, this technique remains,
arguably, the most reliable. The challenge, however,
is to translate the observations recorded at a gauge
to a survey area that is at a remote distance from
this location. Tidal errors will increase with distance
from a gauge, and the problem is to quantify and to
control these errors.
The accuracy requirements for a bathymetric survey
are often specified in terms of a percentage of water
depth. Hydrographic surveys, for nautical charting,
usually follow the criteria defined in the International
Hydrographic Organisation (IHO) S-44 publication
(International Hydrographic Organisation, 1988).
These criteria are given below:
Special Order: 0.25metres + 0.75% of depth
Order 1: 0.5 metres + 1.30% of depth
Order 2&3: 1.0 metres + 2.3 % of depth
These figures are given for the 95% confidence level.
Using these criteria, the resultant Depth versus Accuracy
graph for shallow-medium water depths is
shown in Figure 1.
The theory of using GPS as an alternative
method of deriving tide is introduced, and
this includes a brief description of the different
GPS techniques. The concept of accuracy
versus precision is discussed in relation
to the definition of tidal datums.
In order to assess the performance of GPS
for tidal modelling a trial was organised, and
a number of GPS systems were installed
on a survey vessel. The trial is described
here, the results are presented, and some
of the technical and logistical problems are
discussed. Examples of GPS derived tides
are presented from other survey projects
that have been undertaken.
Finally, some concluding remarks are made,
with suggestions for further investigations.
Figure 1: IHO S44 accuracy specification versus depth.
Traditional Techniques
The traditional method of tidal reduction is to record
the rise and fall of the tide using a gauge. In situa-
Special Order surveys are usually limited to port
areas and their approaches, and it is the Order 1
specification that is most applicable to offshore continental
shelf areas such as the North Sea. From
the specification given above it may be seen that an
60

INTERNATIONAL HYDROGRAPHIC REVIEW
Gardline have produced a digital
model of this chart, which
allows interpolation of MSR
and MHWI values from a series
of "nodes" that surround the
vessel location. If desired, a
weighted mean may be derived
by combining data from a
number of ports. The seamless
digital model allows the smooth
interpolation of tidal values as
a vessel transits a large survey
area (Figure 3).
Figure 2: BA5058 co-tidal chart.
When the co-tidal approach is
used for tidal modelling, knowledge
of the accuracy of the cotidal
chart is required in order
to compile the error budget. The
accuracy of BA5058 varies with
location, but is usually quoted
as:
Mean Spring Range:
0.5metres
Mean High Water Interval:
30 minutes
Figure 3: Digital version of B5058, illustrating model grid nodes and vessel track.
accuracy of approximately 0.8m (95%) is required in
a water depth of 50m.
Extrapolation of tides from a gauge to the survey
area is typically achieved using either a hydrodynamic
model or a co-tidal chart. In the North Sea,
Admiralty co-tidal publications BA5057/5058/5059
are widely used for this purpose (Figure 2).
The co-tidal chart presents contours of Mean Spring
Range (MSR) and Mean High Water Interval (MHWI).
The influence of these factors
on the modeled tidal height is
a function of tidal range and
period, and will vary across a
large survey area. Experience
has shown that generally these
are conservative estimates,
but it is also true that in some
instances the model may not
be sufficient to meet survey
requirements. On these occasions,
it is possible to make local
improvements to the model
by deployment of one or more tide-gauges at the
survey area. Collection of tidal information for a
one-month period is sufficient to enable harmonic
analysis to extract the major tidal constituents, and
these may then be used to quantify, and potentially
also improve, the accuracy of the co-tidal model
(Figure 4).
Unfortunately, the deployment of tide-gauges in offshore
areas is accompanied by significant risk of
loss, typically of the order of 30-40%. It is not vi-
61

INTERNATIONAL HYDROGRAPHIC REVIEW
them to be useful as tides,
detailed knowledge of the relationship
between the GPS
height and the tidal datum
is required. Only with this
knowledge can the GPS derived
tides be described as
accurate.
However, notwithstanding
this comment, precise GPS
observations may provide
useful information on tidal
range and period, even with
incomplete knowledge of
Figure 4: Comparison of offshore tide-gauge and equivalent tides derived from onshore
gauge and co-tidal model.
absolute datum. In this respect
GPS offers potential
to be used as an aid to the
able, therefore, to deploy a number of gauges for
the period of an extensive survey with an expected
traditional approach by helping to identify scale and
phase errors in a co-tidal model.
duration of several months. Hence, there are major
cost and logistical advantages in seeking alternative
tidal strategies.
GPS Techniques
In the near-shore environment certain GPS solutions
have become widely used and accepted techniques
for horizontal control and for use in tidal modelling.
These solutions are limited by the requirement for
close proximity to a reference station, hence their
restriction to near-shore surveys.
In recent years, advances in GPS techniques and
processing algorithms suggest that other GPS methods
may deliver height solutions of sufficient accuracy
for use in tidal modelling.
The GPS solution is referred to the GPS datum, and
height is reported as an ellipsoidal height above the
WGS84 ellipsoid (the complexities of the current
"GPS datum" and ellipsoid are not discussed here).
In contrast, tidal heights have historically been referred
to a local tidal datum, known as Chart Datum,
or derivatives thereof, such as Lowest Astronomical
Tide (LAT).
The suitability of GPS derived tides, therefore, is
determined by the stability and repeatability of the
GPS height solution, but also by the availability of an
appropriate transformation of vertical datum and an
assessment of the associated error.
Current GPS Solutions
However, before considering this in detail, it is necessary
to refine our concept of accuracy by introducing
the distinction between accuracy and precision.
These are not unfamiliar terms in the field of
positioning, and the importance of the distinction
is often discussed in relation to geodetic datums.
Thus, a GPS receiver is capable of delivering a position
of high precision or repeatability, but if the user
has incomplete or erroneous knowledge of the appropriate
geodetic datum transformation, then the
final coordinate could be of low accuracy.
In an analogous situation, GPS derived heights
may be very repeatable or precise, but in order for
High-accuracy GPS positioning techniques that are
potentially suitable for GPS tides may be divided into
four categories.
Real-Time Kinematic (RTK)
RTK is established as an acceptable method for deriving
GPS tides. The RTK method relies on real-time
transmission of carrier phase observations from a
local reference station. The method is capable of
producing high precision GPS height observations,
but is limited in range, and therefore ideally suited
to local near-shore surveys, such as harbour and
port approaches.
62

INTERNATIONAL HYDROGRAPHIC REVIEW
In coastal studies, the integration of land-based and
offshore survey data is increasingly important, and
the adoption of the land-based vertical datum for
use offshore is a simple method to achieve this integration.
The RTK method is most suited to this
approach, as derived GPS heights will be referred to
the height of the onshore reference station, at which
the precise height in the desired vertical datum may
be derived.
An extension of the RTK method, which overcomes
some of the logistical problems associated with
operating within limited range of a reference station,
is Network RTK. These are usually commercial networks
to which a user may subscribe.
Post-Processed Kinematic (PPK)
PPK, as the name implies, is not a real-time solution.
The technique involves the post-processing of data
collected on the survey vessel, with data acquired at
one or more reference stations. If precise positioning
is not required in real-time, then this method offers
significant advantages. If a public GPS service
exists which can supply appropriate reference station
data, then the PPK method obviates the need to
establish local reference stations.
The RTK and PPK techniques generally give a height
solution with centimetre-level precision when working
within approximately 10km of a reference station.
Typically, the maximum quoted range to a reference
station is 20km.
Globally Corrected GPS (GcGPS)
The RTK and PPK techniques are only suitable for
work in close proximity to reference stations. In order
to potentially overcome these limitations, a number
of commercial operators now offer a high-accuracy
Globally Corrected GPS (GcGPS) service. These systems
have global coverage, based on large investment
in infrastructure to support the service. There
are four major commercial systems available today:
Fugro: Starfix HP
Fugro: Skyfix XP
C&C Technologies: C-Nav
Veripos: Veripos Ultra
Two distinct approaches are employed to derive
these high-accuracy solutions. Skyfix XP C-Nav and
Veripos Ultra systems are largely based on a similar
technique, whereby a network of monitoring stations
determines satellite orbit and clock corrections, and
these corrections are broadcast to the user.
The Starfix HP service is more analogous to the familiar
Differential GPS (DGPS) service, as reference
stations are used to derive corrections for GPS observables,
which are then processed and optimised
as a high-accuracy Wide-Area DGPS (WADGPS) solution.
Typically, these systems claim a vertical accuracy of
better than 30cm (2DRMS).
Precise Point Positioning (PPP)
The final high-accuracy GPS technique discussed,
is the Precise Point Positioning (PPP) method. This
is a post-processed solution, which in contrast to
the PPK method, does not require data from reference
stations. The technique uses precise GPS orbit
and clock corrections in conjunction with raw GPS
data logged on the survey vessel. In this respect
the method is similar to the real-time GcGPS technique.
However the PPP method is unique in that
neither a real-time correction service, nor reference
station data is required for the solution. The user,
however, does require access to the Internet in order
to download the precise orbit and clock parameters,
and there is some delay before this information is
published.
The software tested on these trials was TerraPOS,
produced by Terratec of Norway. Accuracy specifications
are dependant not only on GPS geometry
but also on the duration of the observation period.
Figures of 40cm (2VRMS) are quoted for a 1-hour
period, reduced to 8 cm (2VRMS) with 24 hours of
data. To process data from a high-dynamic environment,
such as a vessel at sea, raw GPS data should
be recorded at a frequency of 1Hz.
GPS Dynamic Positioning Trials
In order to investigate the performance of the different
GPS techniques described above, a small trial
was organised so that systems could be compared
under the same (or very similar) set of operational
conditions.
The trial took place in March 2007, at Great Yarmouth,
Norfolk, UK, and utilised the Gardline nearshore
survey vessel, MV Confidante (Figure 5). The
63

INTERNATIONAL HYDROGRAPHIC REVIEW
each of the GcGPS
systems, a temporary
scaffold mounting
was installed on
the starboard side of
the vessel (Figure 6).
This arrangement was
used for installation
for all GcGPS antennae.
In practice it was
found that one system
(nearest to the bridge
superstructure), did
suffer from some
masking, but this did
not detract significantly
from the trials.
Figure 5: Near-shore survey vessel MV Confidante.
following systems were employed for the trial:
• 2 x RTK systems
- A local system established in the port area
especially for the trials.
- A Network RTK system, part of the Leica Smart-
Net system.
• 4 x GcGPS systems
- Starfix HP
- Skyfix XP.
- C-Nav.
- Veripos Ultra.
The temporary RTK
system comprised a
pair of TopCon Hyper
RTK receivers, with
UHF data link, and
the reference station was established on a Gardline
building within the port area.
The second RTK receiver was a Leica GX1230 receiver,
used in conjunction with the national Smart-
Net service, operated by Leica in conjunction with
the Ordnance Survey (OS). This service uses the OS
network of GPS base stations (OS Net) and real-time
corrections are delivered using GSM or GPRS technology.
The second reference station was established on
another Gardline building, and this was occupied
• 2 x Reference Stations
- A local station, established on Gardline premises
as RTK base station.
- A local station, established on Gardline premises
as PPK base station.
• 2 x Permanent Tide-Gauges, both operated by
the Great Yarmouth Port Authority
- A gauge at the river mouth.
- A gauge at the first river crossing (Haven Bridge).
• 1 x Temporary Tide-Gauge
- Deployed offshore during trials.
In order to attempt identical site conditions for
Figure 6: GPS antennae installations onboard MV Confidante.
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INTERNATIONAL HYDROGRAPHIC REVIEW
with a Trimble MS750 dual-frequency receiver. This
station was configured to log data at 1 Hz.
The purpose of the trials was not to compare or
contrast specific systems, but to investigate the performance
of generic system types with respect to
the precision of the height information. Overall accuracy
of the systems, a function of vertical datum
transformations, was not investigated.
It was the intention to log data from all the systems
continuously for four days; two of which were to be
spent offshore. Unfortunately, poor weather intervened
and offshore "dynamic" trials had to be severely
curtailed. Real-time position information was
acquired using the standard Windows tool, Hyperterminal,
to log NMEA-0183 datagrams at a frequency
of 1 Hz. In addition, each GPS system logged raw
data, in their own proprietary format.
Data Processing and Results
Position solutions, whether acquired in real-time or
by post-processing of raw GPS data, were processed
in the same manner. In general, the position solution
was either the NMEA GGA datagram recorded
using Hyperterminal, or a similar text file output by
one of the GPS post-processing packages.
It will be noted that no attempt has been made in
the trials to compensate the GPS height solutions
for vessel motion. In a highly dynamic offshore environment
this obviously introduces additional noise
into the measurements that requires appropriate
smoothing and/or filtering. The procedure adopted
was simple in concept:
- Data, logged at a 1-second epoch, was initially
processed using a 2-minute Median filter. The Median
value was chosen in preference to the Mean
in order to attempt to mitigate the influence of
isolated outliers.
- Median values were smoothed with a 10-minute
filter.
Due to lack of motion data, it is possible for small
biases to enter the processed solution. Generally,
heave motion should have a zero mean, but using a
simple mean for observations subject to pitch and
roll motion will introduce small errors, dependent on
the antenna height above the centre of motion. In
these trials these potential errors are assumed to
be insignificant.
Results from these trials are presented in the Figures
that follow.
Figure 7. GPS height derived from the four GcGPS
systems over a 5-day period. Note that individual
systems are not identified, but labelled from A to D.
What is clear from this display is that similar precision
has been attained by all systems.
Figure 8. A 3-day period of data, as the vessel was
alongside the quay, and within a short distance of
one of the permanent tide-gauges. The GPS data
has been arbitrarily shifted to match the tide-gauge
datum. This data confirms precision of all the systems
within manufacturers specifications.
Figure 7: GcGPS ellipsoidal height comparison.
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INTERNATIONAL HYDROGRAPHIC REVIEW
Figure 8: GcGPS heights versus tide-gauge.
Figure 9: GcGPS heights versus RTK heights versus tide-gauge.
Figure 9. A 7-hour period of data obtained whilst the
vessel was offshore. This includes data from the two
RTK systems and the temporary tide-gauge. The tidegauge
data has been arbitrarily translated for comparison
with the GPS data. This data has a number
of points of interest:
- One of the GcGPS systems shows periods of outages.
This was caused by antenna masking, mentioned
earlier, as the vessel was heading in one
direction, and does not reflect on the performance
of that particular system.
- The GcGPS solutions agree closely with the RTK
solutions.
- Although absolute accuracy was not a goal in this
trial, it is interesting to note that one of the RTK
systems closely matches the GcGPS data, and
one shows a difference of approximately 0.5m.
The Leica system, part of a permanent national
network, was the RTK system that agreed with the
GcGPS data, and the temporary system shows the
height discrepancy.
- Within all the GPS systems, there is a slight undulation
apparent in the data at about 1300 hours.
This undulation is not evident in the tide-gauge
data. A second, less pronounced, undulation is
also seen at about 1400 hours. This feature is
interpreted to be the result of the survey vessel
steaming up and down a survey line within the tidal
regime, and thus the measured tidal signature
is slightly different to the gauge data recorded at
a fixed location. It had been the intention to deploy
the temporary tide-gauge at one end of the survey
line, and collect data between this gauge and the
permanent gauge at the river mouth. Logistical
problems precluded this deployment.
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INTERNATIONAL HYDROGRAPHIC REVIEW
Figure 10: RTK heights versus tide-gauge.
Figure 11: PPK raw height versus smoothed height.
Figure 10. A comparison of the 7-hour RTK solution
versus the post-processed solutions. Dual-frequency
GPS data was logged on the vessel every second.
This data was post-processed with data from the
Gardline reference station, also logged at 1Hz, hence
a PPK solution was derived every second. The PPK
solution is shown in black on the graph.
The Precise Point Position solution (PPP) was derived
using TerraPOS software. The same dual-frequency
data used in the PPK processing was used to derive
this solution. The PPP solution is shown in red on
the graph.
The post-processed solutions in this graph appear
much more stable that the real-time GPS solutions,
but this is due to a slightly different processing sequence,
and these datasets have been subject to
higher levels of smoothing compared to the real-time
data. A comparison of the raw and smoothed PPK
solutions is shown in Figure 11.
GPS Accuracy - The Datum Problem
The trials conducted in Great Yarmouth suggest that
GcGPS could be a viable aid in tidal modelling. The
issue of absolute accuracy has not been discussed,
but based on data presented here, the precision appears
to be within the manufacturers quoted specifications.
Without addressing the datum problem, a GPS derived
tide may still be useful as an aid to traditional
tidal modelling, but would have limited value if used
in isolation. The GPS tidal curve may be arbitrarily
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INTERNATIONAL HYDROGRAPHIC REVIEW
moved to match a conventional
tidal curve, and this could be
used to provide confirmation (or
not) of co-tidal range and time differences.
The vertical datum for GPS height
information is the geodetic reference
ellipsoid. To have real
value in tidal modelling a transformation
into a tidal datum is
required. The geodetic framework
is a precise mathematical model,
but the concept of a tidal datum
is an irregular, disparate surface,
based not on mathematics, but
empirical analysis of historical
data. A transformation, therefore,
between the two datums is not a
trivial matter.
Figure 12: Mean Sea Surface (KMS04) compared to geoid (EGM96) in the North
Sea.
The relationship between different vertical reference
datums is the subject of on-going research. In the
UK, the Integrated Coastal Zone Mapping project
(ICZMap), investigated the problem of integrating
land and hydrographic survey data (Adams, 2004).
This highlighted the desirability of a uniform vertical
reference frame, and subsequently the UKHO has
commissioned a research project to investigate the
feasibility of a Vertical Offshore Reference Frame
(VORF) for use in UK waters (Adams et al, 2006).
Notwithstanding the complexities of the issues, the
transformation of GPS heights into a tidal datum can
be achieved with some success in a simple threestage
procedure.
1. Reduce the GPS antenna height, relative to the
ellipsoid, to the Vessel Water Line. Usually two
static measurements are required for this;
GPS antenna height relative to the vessel reference
point.
Water line relative to the vessel reference
point.
2. After this correction we have a measurement of
instantaneous sea level above the geodetic ellipsoid.
The next stage is to transform into a tidal
datum, and the obvious choice is Mean Sea Level.
A global geoid model could be used as an approximation
of MSL, and the most recent model that is
readily available is EGM96. This model has been
used in the examples here. In future work this will
68
be replaced by a Mean Sea Surface (MSS) model.
The latest version that is readily available is
KMS04, however this is shortly to be replaced by
KMS06, recently renamed DNSC07. In the North
Sea area this model should offer significant improvements.
Figure 12 illustrates the difference
between EGM96 and KMS04, which can attain
levels of 50 cms in this area.
3. The final stage is the transformation from MSL
to the required tidal datum. Nautical charts have
traditionally been related to a local tidal datum,
known as Chart Datum, which in the UK usually
approximates to Lowest Astronomical Tide (LAT).
The local nature of these individual datums gives
rise to the problem of integrating these into a national
(or international) model. In the future, within
UK waters, the VORF project will potentially provide
the necessary tools to resolve this issue. In the
absence of such a model, an effective solution is
to use the co-tidal approach. The co-tidal chart is
used in the traditional approach to translate tides
from a local port to the survey area. In this process
the tidal datum is implicitly translated. The
same method, therefore, may be used merely to
translate the datum.
Further Studies
Further studies continue into the use of GPS for tidal
monitoring. In the near-shore environment, RTK and
PPK solutions routinely provide accurate and reliable
data for tidal reduction. In these littoral areas, where

INTERNATIONAL HYDROGRAPHIC REVIEW
the fusion of land and hydrographic survey data is important,
the datum problem may be overcome by the
adoption of the vertical land datum for use at sea. In
the UK, Ordnance Datum Newlyn (ODN) is frequently
used for this purpose (Figure 13).
In the offshore environment, GcGPS systems continue
to be evaluated. The evidence at the moment suggests
that performance within system specification,
as achieved on the trials and illustrated in Figures
7 and 8, is difficult to achieve with 100% reliability.
It is acknowledged, however, that system specifications
do not quote figures with 100% reliability, and
no attempt has been made to quantify a "reliability"
figure.
Of more importance is to investigate the reasons for
the observed GPS outages. Possible causes include
poor GPS geometry, multi-path or other local interference,
or perhaps excessive vessel motion as a result
of poor weather. Figures 14 and 15 contrast the same
system in periods of contrasting weather conditions.
In these images the blue data represents the raw
GPS height, and the spread of data is indicative of
the vessel motion. The purple curve represents the
smoothed data. At this stage no firm conclusions are
drawn from this data, as other influences, such as
satellite geometry have not been investigated.
Figure 16, from a different dataset, is included to
show the influence of poor geometry, in periods of
both good and bad weather. The red curve indicates
the satellite geometry, as reported by the VDOP figure.
The large spikes in the VDOP values clearly influence
the height solution, and the system appears to
take some time to recover from the outages.
Figure 13: RTK surveying in near-shore area; soundings
reduced to land datum (ODN).
Periods of poor geometry still occur periodically,
even in areas such as the North Sea. Figure 17 illustrates
predicted GPS geometry from a day in June
2007. The outage in the early morning, particularly
evident in the geometry plot, persisted for a number
Figure 14: GcGPS data; good weather, good reliability
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INTERNATIONAL HYDROGRAPHIC REVIEW
Figure 15: GcGPS data; poor weather, poor reliability GPS mode (red) falls from 4 to 1 when solution fails.
Figure 16: GcGPS data with 1 metre jumps in height solution; VDOP (red) indicates periods of poor geometry.
of weeks. GcGPS data for this period has yet to be
analysed, although Gardline vessels did report minor
difficulties with DGPS solutions.
Conclusions and Suggestions for Future
Work
A number of high accuracy GPS techniques exist
which may potentially be used in tidal modelling. In
the near-shore environment RTK and PPK solutions
are accepted methods. Trials suggest that GcGPS
systems may provide the necessary precision when
working offshore, and that the post-processed PPP
technique could be viable if a real-time solution is
not required.
GcGPS data acquired in the offshore environment
has not managed to replicate the reliability obtained
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INTERNATIONAL HYDROGRAPHIC REVIEW
in the near-shore trials. Further work is necessary to
identify the causes of GPS outages.
Analysis of offshore data is more problematic, as generally
there are no benchmark systems with which to
evaluate results. Deployment of offshore tide-gauges
would be a suitable baseline system, but due to risk
of loss this rarely commercially viable. Where a suitable
survey grid is proposed, analysis of bathymetric
mis-ties may provide information on tidal precision.
References
International Hydrographic Organization (1998)
IHO Standards for Hydrographic Surveys, International
Hydrographic Organization, Special Publication
No. 44, 4th edition, April, pp23.
Adams R. (2004). Seamless Data and Vertical Datums
- Reconciling Chart Datum with a Global Reference
Frame. Hydrographic Journal, 113, 9-14.
Adams, R. Iliffe, J. Ziebart, M. Turner, J. Oliveira,
J. (2006). Joining
up Land and Sea
- UKHO/UCL Vertical
Offshore Reference
Frame. Hydro International,
December, 7-9.
Acknowledgements
Figure 17: GPS satellite visibility and geometry in the North Sea, June 2007.
Refinements to the filtering and smoothing procedures
are to be investigated in conjunction with
the use of motion sensor data for compensation of
antenna motion. Investigation into the relationship
between GPS solution, vessel motion, and GPS geometry
may help determine any weather dependency
of the systems.
The concept of tidal accuracy, as it relates to vertical
datums, has been discussed. In the future it is
likely that all heights will be referred to a GPS ellipsoidal
datum, and a bespoke model will manage the
transformation between this and traditional tidal datums.
Until such times, GPS ellipsoidal heights may
be transformed to a tidal datum using existing Mean
Sea Level and co-tidal models.
The author would like
to express his appreciation
and gratitude
to the following companies
for the supply
of equipment and assistance
on the GPS
trials: Fugro Survey,
C&C Technologies and
Veripos. Also many thanks to Terratec for assistance
with PPP processing.
Biography of the Author
Dave Mann is a graduate of the University of Nottingham
with a Masters degree in Geodesy, and
has been involved in various aspects of land and
hydrographic surveying for 25 years. For most of his
career he has been employed by Gardline, initially
as a field surveyor, later as Assistant Chief Surveyor,
now as Survey Support Manager, responsible for the
development, integration and support of survey systems.
E-mail: dave.mann@gardline.com
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Note
The WEND Concept for a Worldwide ENC Database
- Past or Future
A Review of Progress and a Look to the Future
Horst Hecht, Federal Maritime and Hydrographic Agency (BSH), Hamburg/Rostock
(Germany); Abri Kampfer, Hydrographic Office, Cape Town (Republic of South Africa); Lee
Alexander, CCOM-JHC, University of New Hampshire, Durham, New Hampshire (USA)
Introduction
Over 13 years ago, the IHO developed
a concept, called "Worldwide Electronic
Navigational Chart Database" (WEND),
to:
"ensure a world-wide consistent level
of high-quality, updated official ENCs
through integrated services that support
chart carriage requirements of SOLAS
Chapter V, and the requirements of the
IMO Performance Standards for ECDIS"
(see Annex)
To help implement the concept, a WEND
Committee was established to:
"promote the establishment of a Worldwide
Electronic Navigational Chart Database
(WEND) suitable for the needs
of international shipping". (see WEND
Committee Terms of Reference)
While significant progress in ENC data
production has been accomplished,
market take-up of ENCs remains below
expectations, and substantial additional
efforts are still necessary to
ensure worldwide coverage. Some are
now questioning the value of the WEND
concept. The purpose of this paper is
to review the development of the WEND
concept, discuss some of the more
important factors that have impeded a
more rapid progress by WEND, and to
provide some recommendations on the
best way forward.
Background and Objective of
WEND
It was in 1985 that IMO and IHO first
initiated discussion on the development
of the Electronic Chart - or more
precisely, on what eventually became
the "Electronic Chart Display and Information
System" (ECDIS). In 1988,
IMO requested that IHO study how distribution
of the data needed for ECDIS
should occur, and how the "Electronic
Navigational Chart" (ENCs) and its updating
could be organised. The result
was a blueprint of an organisational
scheme that was first published by
IHO as "Updating the Electronic Chart"
(IHO S52Appl, 1996). However, at
that time the standardisation of ECDIS
was far from complete. Thus, in 1993
(four years before completion of ECDIS
standardisation) IHO began considering
how it as an organisation could cope
with the challenges of the electronic
age. At that time, no IHO Member State
had any idea of the complexity of the
task, particularly the amount of effort
required to produce ENCs, and to develop
concrete technical and organisational
arrangements.
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INTERNATIONAL HYDROGRAPHIC REVIEW
One of the first steps that IHO undertook was to
define the overall objective and the necessary principles
required to produce and make ENCs available
worldwide. The goal of this effort was implied in its
title: "Worldwide Electronic Navigational Chart Database"
(WEND). This name reflects two mutually-dependant
conditions:
In principle, every national HO is responsible for
producing the ENCs of its national waters. It is
holder of copyright and also liable for the data.
This requirement was an outcome of the XlVth IHO
Conference where the ownership of all data collected
by an HO in its national waters has been
defined, irrespective of its uses (for example, the
data in charts, in nautical publications, in other
publications and data bases).
In order to provide an authoritative and unambiguous
product, the totality of ENCs contributed from
national HOs must be considered as a distributed,
"virtual" data base (i.e., WEND), from which a
physical data base can be assembled. This task
was intended to be performed by Regional Centres
(called RENCs) along with the necessary centralised
quality assurance.
The expectation was that WEND would become the
cooperative measure of all IHO members to ensure
worldwide availability of ENCs. This need for cooperation
is not only a logical consequence of the nature
of worldwide ENC coverage as a distributed database;
it is also one of the purposes of the IHO itself. This
is clearly stated in the IHO Convention in Article IIb:
"to bring about the greatest possible uniformity in
nautical charts and documents". It is covered even
more specifically in Sections 3 and 4 of Regulation 9
of the new Chapter V of the SOLAS Convention:
"3 Contracting Governments undertake to ensure the
greatest possible uniformity in chart and nautical
publications and to take in account, whenever
possible, relevant international resolutions and
recommendations.
4 Contracting governments undertake to co-ordinate
their activities to the greatest possible degree in
order to ensure that hydrographic and nautical information
is made available on a world-wide scale
as timely, reliably, and unambiguously as possible."
(IMO SOLASV, 2000)
As pointed out by Ehlers (Ehlers, 2002), unlike the
IHO Convention, SOLAS V is "binding under international
law." Under the new Regulation 9, the govern-
74
ments which are parties to SOLAS are now required
to maintain hydrographic services. In addition to
providing all nautical information necessary for safe
navigation, they are required to cooperate and to carry
out nautical and hydrographic services including
data management services. Since almost all international
voyages transit through the coastal waters
of different nations, this has become a fundamental
reason for IHO members to cooperate in terms providing
worldwide ENC coverage and distribution.
Current Status of WEND
In accordance with the principles in the IHO and the
SOLAS Conventions, WEND was conceived as the
means of achieving the necessary cooperation to
ensure worldwide uniformity and availability of ENCs.
In order to define the terms of this cooperation, essential
principles were drawn up. These so-called
WEND Principles (see Annex) have been refined and
clarified in recent years in the light of further experience.
They are now part of the compendium of IHO
Technical Resolutions; see Resolution K 2.19 (IHO
M3, 2007).
But, have they proven successful In many respects,
it is the dilemma of whether the glass is either halffull
or half-empty. One way to address this question
is to examine where IHO stands now in terms of ENC
availability and services. The following criteria are
provided by the WEND Principles themselves.
Worldwide coverage and availability including updating
(WEND Principles 1.1 and 2.3); ENC Coverage
has greatly improved in recent years, but there
are still some significant gaps. Europe, USA and
East Asia are well covered, at least as far as major
shipping routes and ports are concerned. However,
important areas in Southeast Asia are still missing.
Coastal Africa and South America have very limited
coverage. Nevertheless, a recent study undertaken
by the Norwegian Classification Society Det Norske
Veritas (DnV) reached the conclusion, based on
an analysis of 11 important international shipping
routes, that by 2010 there will be sufficient coverage
to justify an ECDIS carriage requirement for
all vessels of at least >10,000 gross tonnage (oil
tankers >3,000GT), and for new vessels >3,000GT
(oil tankers >500GT) [5]. While this analysis may
be valid for the category of larger vessels with its
limited number of destinations, there is still some

INTERNATIONAL HYDROGRAPHIC REVIEW
doubt whether sufficient ENC coverage will exist for
the greater number of smaller ports and their approaches
that are frequented by tankers of only
500 GT. In terms of ENC availability and updating
services, there are also a number of regions
where ENCs have been produced, but for a variety
of reasons, they are not yet widely available or not
updated. Most often, this is because the producer
HOs have not joined a RENC.
Integrated services (WEND Principles 1.2,1.4 and
1.7); As defined by WEND, integrated ENC services
are made possible through the Data Servers, as
described in the IHO Data Protection Scheme S-
63 (IHO S63, 2003), (IHO WEND, 2006): ENCs are
supplied centrally to the Data Servers from RENCs
who cooperate closely on joint S-63 implementation
and consistent business terms. Currently, the
two RENCs (IC-ENC and PRIMAR-Stavanger) have
a combined membership of 35 HOs. Cooperation
between these two RENCs means that most of the
existing ENCs are available today through integrated
services. Additional ENCs are also made available
through bilateral arrangements between the
Data Servers and some non-RENC HOs. Service
integration requires, in practical terms, that each
Data Server is given control over the definition of
the cell permits that are created, and which effectively
control the subscription licence that a customer
has for each ENC. Where an HO implements
S-63 itself for its own ENCs, particularly data
signing and cell permit generation, the provision
of full route coverage for an international voyage
via an integrated service is therefore impossible to
achieve. Unfortunately, there are a small number
of HOs that are doing this, and distributing their
ENCs independently of the Data Servers.
User-friendliness of services (WEND Principle 1.7);
User-friendliness was included as a principle under
WEND to help promote the use of ENCs. Apart
from service integration (see above), one particularly
relevant feature of user-friendliness is pricing.
The pricing policies of IHO members vary from nocost
(such as USA) to high prices well above the
equivalent cost of paper or raster charts. Whilst
the headline cost of an ENC cell is often seen to
be the same as that of a paper chart (i.e. about
$25-$30), it has to be remembered that an ENC
cell very often provides much less data coverage
than the equivalent paper chart which has overlaps,
and inset plans which are normally sold as
separate ENC cells. The price of an ENC cell also
typically covers only an annual licence, whilst the
price of a paper chart is for the life of that edition
of the chart, and so tends to be re-purchased less
frequently. The result is that the WEND database
is projected to be more expensive for a customer
than an equivalent coverage in paper. It is also considerably
more expensive than the commercial ECS
portfolios that already exist. This therefore makes
the ENC/ECDIS option much less attractive either
to resellers or to users. Strategies to overcome the
unfavourable ENC pricing are under consideration,
such as "Pay-Per-Use" (IHO WEND/ESF, 2006), but
such schemes require a new and more flexible system
of licensing and so are impossible as long as
HO participation and RENC cooperation and policy
is less than universal. In other words, a partial
implementation of WEND principles will never be
enough to resolve this important issue.
Greatest possible standardisation, consistency, reliability
(WEND Principle 1.3); A significant source
of inconsistencies between ENCs is the differing
interpretation and use of the IHO Data Transfer
Standard (IHO S-57) (IHO S57, 1996), and its lack
of a prescriptive ENC product specification. Whilst
ENC validation tools can detect formal errors in
data structure and formatting, they cannot detect a
missing object or an inappropriate attribute or portrayal
instruction. This sort of inconsistency largely
exists with ENCs from different producers, who
interpret the standard, and in particular the Use
of the Object Catalogue, differently. To help overcome
this problem is another reason for an HO to
participate within a RENC since it acts as an independent
quality assurance body and thereby standardises
all the ENCs under its control. The lack of
a definitive product specification is also a major
source of inconsistency, allowing different HOs to
compile their ENC products in different ways (e.g.
the setting of compilation scales) resulting in poor
results when displayed on a typical ECDIS. This introduces
the risk that only a series of datasets are
produced rather than a database product suitable
for primary navigation. This risk is increased due
to the fact that most ENCs have been produced
by digitizing paper charts which were themselves
designed to be used individually rather than as
part of a database. Paper chart schemes therefore
generally contain overlaps, are of different ages
(and so contain different information), and are produced
to varying scales. Consequently, when put
together, they fail to offer seamless coverage. Discrepancies
between adjacent ENC cells that were
undetected on paper charts become immediately
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INTERNATIONAL HYDROGRAPHIC REVIEW
obvious on an ECDIS screen. The problem not only
occurs within a single paper chart series, but is
also particularly pronounced on national charts
that border charts from neighbouring HOs. The
need for ENC harmonization within each national
ENC series and between adjacent producer HOs is
well recognized, and is a key function that is performed
by the RENCs. Unfortunately, current RENC
membership makes up for less than 50% of IHO
membership and several of the largest HOs do not
participate in a RENC at all, despite WEND Principle
1.3. This means that the percentage of ENC
coverage administered through RENCs is even
less in terms of sea area.
Looking Ahead
Overall, a serious challenge related to WEND continues
to be sub-optimal ENC coverage. This is confirmed
by feedback from shipowners who indicate
their continuing preference for official ECDIS data
- provided there is sufficient route coverage. In view
of proposals to make ECDIS and ENCs mandatory
for SOLAS vessels, overcoming this problem must
be given the highest priority. IHO, at its recent XVIIth
Conference, referring to the DNV study (DNV, 2007),
has expressed its support to IMO and has resolved
to provide the required ENC coverage by 2010 (Decision
21 of IHC XVII) (IHO ConfDecisions, 2007).
Although this is part of the IHO 2008 to 2012 Work
Program, it may not be possible for some HOs to
complete sufficient ENC coverage using their own
resources. In this regard, it will be necessary for
these HOs to consider entering into bilateral ENC
production arrangements with other HOs who have
sufficient ENC production capability to provide assistance.
The other challenges with ENCs can be best characterised
as problems of service quality. It is very
important to make a clear distinction between "service
quality" and "data quality". In the wider sense,
service quality relates to the relationship between
the distributor and the end user. This includes data
presentation (i.e. the quality of the data as a navigational
product), pricing, packaging, and distribution
options. Meanwhile, data quality relates to correctness,
completeness, and being up-to-date. Data
quality deficiencies could have a direct impact on
safety-of-navigation. In principle, official ENC data
will always be superior in terms of data quality
(both correct and complete), and more up-to-date
compared to commercial data derived from official
products.
The display of ENC data on ECDIS requires careful
attention, as the suitability and consistency of
encoding decisions for each dataset within the database
will affect its fitness for purpose to support
primary navigation. Unfortunately ENC data will suffer
in comparison to commercial electronic chart
data since the inconsistencies between adjacent
ENC cells are readily apparent. For most commercial
data, such data inconsistencies will have been
superficially "cleaned up" by the commercial data
producers, and they will have schemed their product
database in a consistent manner and to a single
product specification. Of course this does not
improve the data quality, and the "cleaning" could
have safety consequences as any adjustments will
have been made without reference to the original
source data or to the HOs publishing the source
chart. On the other hand, an inconsistent chart
image displayed on a screen may be confusing for
the mariner, and result in unnecessary and frustrating
manipulation of the ECDIS settings in order to
retain a consistent presentation as he pans across,
and zooms into, the database. It is important, therefore,
that HOs work together to remove any inconsistencies
from their data to achieve both high data
quality and a clean display.
It is worth noting that almost all of the issues that
fall into the class of "service quality" can be solved
by better international cooperation. Indeed, if the
WEND Principles were strictly followed, and if all HOs
were to cooperate with a RENC, problems would be
jointly addressed and resolved. In other words, it
is not the WEND concept that has failed, as it is
frequently claimed. Instead, too many IHO members
have not complied with the WEND Principles that
the Organization adopted more than 10 years ago.
In our view, WEND is not the problem. To the contrary,
it has the potential to be the solution.
In what appears to be a promising sign, the IHO
at the recent XVIIth Conference repeated its commitment
to the WEND Principles, and has stressed
that it is the responsibility of its Member States and
Regional Commissions to improve coverage and
consistency (IHC Decision 20) (IHO ConfDecisions,
2007). But for WEND to be successful, some changes
in approach and organisational structure should
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INTERNATIONAL HYDROGRAPHIC REVIEW
be considered.
In our view, it is debatable whether the current WEND
structure of two RENCs is ideal. The first blueprint of
the WEND structure dates back to a time when modern
broadband communication allowing rapid data
exchange of high data volumes was not readily available.
Hence it was thought that a regional structure
should be the basis for facilitating cooperation. The
fact that the two RENCs have a worldwide scope, but
they are both situated in the same region (northern
Europe), may illustrate best the problem with the current
WEND structure. However, a regional sub-structure
may still be sensible for organisational reasons.
In this regard, one RENC (IC-ENC) has already developed
such a sub-structure for its 25 members from
five continents.
Despite good cooperation between the two RENCs,
they have a different organisational set-up and different
distribution concepts. Whereas IC-ENC is a
pure RENC concentrating on quality assurance and
leaving service delivery to Data Servers, Primar-Stavanger
is both a RENC and a Data Server. These two
different distribution concepts create confusion, and
are probably not the best way to convince a hesitant
or confused HO to join a RENC.
IHO must acknowledge that in taking responsibility
for producing ENCs the organization and its members
have also taken on a role that requires coordinated
professional management and an operational
service orientation. In order to meet its short and
medium term commitments regarding ENC coverage
and service quality, the IHO must ensure that
it has mechanisms that ensure that decisions on
matters that have worldwide impact (like operating a
"WEND") are acted upon in accordance with the time
schedules and rules that were jointly agreed. After
all, SOLAS V as binding international law ultimately
forces IHO to act decisively. As stated by Ehlers (Ehlers,
2002).
"... the crucial factor regarding implementation of
these commitments is that SOLAS refers to the relevant
decisions and recommendations of IHO, which
are thus connected with the SOLAS Convention. In
this way, the IHO decisions take on a new quality under
international law, namely that of generally recognized
rules and standards as referred to in Articles
211 and 219 of the Convention on the Law of the
Sea. They must be taken into account whenever possible
- that means as a matter of principle - and can
no longer be disregarded on the grounds that they
are not binding. This clearly enhances the value of the
IHO functions".
Recommendations
- It is imperative that IHO members complete ENC
coverage without further delay. For sea areas
where no active HOs exist, IHO should develop a
detailed program that includes a time schedule.
Where necessary, IHO should designate producer
HOs to fill in any remaining gaps in ENC coverage.
- It is crucial that the service quality related to ENCs
be improved. This cannot be achieved without all
HOs cooperating very closely, and this can be best
achieved by participating in a RENC. The WEND
Principles should be followed by all HOs as the
means of fulfilling their obligations under international
law "to ensure the greatest possible uniformity
in chart and nautical publications" and "to
co-ordinate their activities to the greatest possible
degree" (Chapter V SOLAS Convention, Regulation
9). IHO must work with Regional Commissions and
the WEND Committee to develop a plan for full implementation
of the WEND system.
- Bearing in mind that all RENCs need to cooperate
anyway and that there is no real need for strictly
"regional" RENCs, the organisational backbone of
WEND should be further developed and simplified
towards a "Worldwide ENC Coordinating Centre"
(WENC). The purpose is to achieve full participation
of all IHO Member States. Ideally, this will be
the goal of IHO in terms of implementing WEND as
the ENC distribution system of the future.
Concluding Note
Recently, IHB has invited participation in an Extraordinary
WEND meeting to "examine the status of
production of ENCs and the possible problems that
are connected with this", and to discuss possible
solutions. This will be an excellent chance to take
decisions towards full WEND implementation and to
overcome existing problems both in ENC production
and service provision. The preliminary failure at the
recent IMO Safety of Navigation Committee meeting
to find acceptance for a proposal to make ECDIS
mandatory by some later date, and the questions
raised in this conjunction, illustrate the urgency for
IHO to come to a solid and effective solution.
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INTERNATIONAL HYDROGRAPHIC REVIEW
References
DNV (2007): Effect of ENC Coverage on ECDIS Risk
Reduction. Technical Report, DNV Research & Innovation.
Report NO. 2007-0304, Rev. 01. Det Norske
Veritas, April 2007.
Ehlers, P (2002): "Hydrographic Services at the
Crossroads," International Hydrographic Review, Vol.
3, No. 3, November 2002, p.6-13.
IHO ConfDecisions (2007): IHO, Decision #20 -
Resolution Electronic Navigational Chart Coverage,
Availability , Consistency and Quality, International
Hydrographic Conference, May 2007. www.iho.int/
MEM_STATES/CONFDECISIONS18Jul07.pdf.
IHO M3 (2007): Resolutions of the International Hydrographic
Organization. IHO Publication M-3. IHO,
February 2007.
IHO S52Appl (1996): Guidance on Updating the
Electronic Chart. IHO Special Publication S-52, Appendix
1 (Ed. 3). IHO, December 1996.
IHO S57 (1996): International Hydrographic Organization,
IHO Transfer Standard for Digital Hydrographic
Data, IHO Special Publication No. 57 (IHO S-57),
3rd Edition, November 1996, Monaco.
IHO S63 (2003): IHO Data Protection Scheme. IHO
Publication S-63. Edition 1.0, December 2003.
IHO WEND (2006): Some Reflections on the Current
Status of ENC Distribution. Document WEND10-7C.
IHO, September 2006. www.iho.int/COMMITTEES/
WEND/WEND10/WEND10-7C_ENC_Distribution.pdf
IHO WEND/ESF (2006): Pay-for-use ENC pricing.
Presentation by George Arts at the 2nd ECDIS
Stakeholders Forum, IHO Monaco, September
2006. http://www.iho.int/COMMITTEES/WEND/
WEND10/ECDIS_Stakeholders_Forum/ESF06.htm
IMO SOLAS V (2000): International Maritime Organization,
MSC 73/21/Add.2, Annex 7, p. 120-160, 5
Dec 2000.
Annex
Principles Of The Worldwide Electronic
Navigational Chart Database (WEND)
(extract from IHO Res. K2.19)
The purpose of WEND is to ensure a world-wide consistent
level of high-quality, updated official ENCs
through integrated services that support chart carriage
requirements of SOLAS Chapter V, and the requirements
of the IMO Performance Standards for
ECDIS.
1. Service Provision
1.1 Member States will strive to ensure that mariners,
anywhere in the world, can obtain fully
updated ENCs for all shipping routes and ports
across the world.
1.2 Member States will strive to ensure that their
ENC data are available to users through integrated
services , each accessible to any ECDIS
user (i.e., providing data in S-57 form), in addition
to any national distribution or system-specific
SENC delivery.
1.3 Member States are encouraged to distribute
their ENCs through a RENC in order to share
in common experience and reduce expenditure,
and to ensure the greatest possible standardization,
consistency, reliability and availability of
ENCs.
1.4 Member States should strive for harmonization
between RENCs in respect of data standards
and service practices in order to ensure the
provision of integrated ENC services to users.
1.5 Methods to be adopted should ensure that data
bear a stamp or seal of approval of the issuing
HO.
1.6 When an encryption mechanism is employed to
protect data, a failure of contractual obligations
by the user should not result in a complete termination
of the service. This is to assure that
the safety of the vessel is not compromised.
1.7 In order to promote the use of ENCs in ECDIS,
Member States are to strive for the greatest
possible user-friendliness of their services, and
facilitate integrated services to the mariner.
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INTERNATIONAL HYDROGRAPHIC REVIEW
2. Rights and Responsibilities
2.1 SOLAS Chapter V, Regulation 9, requires Contracting
Governments to ensure that hydrographic
data are available in a suitable manner
in order to satisfy the needs of safe navigation.
Once the carriage of ECDIS becomes mandatory,
there will be a consequential requirement
to ensure that such data, as agreed by IMO, are
available in a form suitable for use in ECDIS.
2.2 It is expected that Member States, for waters
of national jurisdiction, will have mature supply
systems for ENCs and their subsequent updating
in place by the earliest date for mandatory
carriage of ECDIS.
2.3 By the dates established by IMO , Member
States will strive to either:
a) Provide the necessary ENC coverage, or
b) Agree with other States to produce the necessary
ENC coverage on their behalf.
IHO will address overall coverage on a regional
basis through Regional Hydrographic Commissions.
2.4 The INT chart system is a useful basis for initial
area selection for producing ENCs.
2.5 Member States are encouraged to work together
on data capture and data management.
2.6 Responsibilities for providing digital data outside
areas of national jurisdictions must be established
(see guidance in Annex).
2.7 Technically and economically effective solutions
for updating are to be established conforming
to the relevant IHO standards. The updating of
ENCs should be at least as frequent as that
provided by the nation for correction of paper
charting.
2.8 The Member State responsible for originating
the data is also responsible for its validation
in terms of content, conformance to standards
and consistency across cell boundaries.
2.9 A Member State responsible for any subsequent
integration of a country's data into a wider service
is responsible for validating the results of
that integration.
2.10 National HOs providing source data are responsible
for advising the issuing HO of update
information in a timely manner.
2.11 Member States should work together to ensure
data integrity, and to safeguard national
copyright in ENC data to protect the mariner
from falsified products, and to ensure traceability.
2.12 In producing ENCs, Member States are to take
due account of the rights of the owners of
source data and if paper chart coverage has
been published by another Member State, the
rights of that State.
2.13 Member States should recognize their potential
exposure to legal liability for ENCs.
3. Standards and Quality Management
3.1 A Quality Management System should be considered
to assure high quality of ENC services.
When implemented, this should be certified by
a relevant body as conforming to a suitable
recognised standard; typically this will be ISO
9001:2000.
3.2 There must be conformance with all relevant
IHO and IMO standards.
3.3 Member States' HOs are strongly recommended
to provide, upon request, training and advice
to HOs that require it to develop their own national
ENC provision.
1 Integrated services are a variety of end-user
services where each service is selling all its ENC
data, regardless of source, to the end user within
a single service proposition embracing format,
data protection scheme and updating mechanism,
packaged in a single exchange set.
2 RENCs are organisational entities where IHO
members have established co-operation amongst
each other to guarantee a world- wide consistent
level of high quality data, and for bringing about
co-ordinated services with official ENCs and updates
to them.
3 The IMO Sub- Committee on Safety of Navigation,
at its 51th Session (NAV 51):
• agreed to recommend to the IMO Marine Safety
Committee the mandatory carriage requirement
of ECDIS for High Speed Craft (HSC) by 1 July
2008.
• did not decide on a mandatory carriage requirement
for other types of ship; this will be considered
in conjunction with a Formal Safety Assessment
(FSA) to be conducted into the use of
ECDIS in ships other than HSC and large passenger
ships.
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INTERNATIONAL HYOROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Note
Relationship of Marine Information Overlays (MIOs)
to Current/Future IHO Standards 1
Dr. Lee Alexander, Chair and Michel Huet, Secretary
IHO-IEC Harmonization Group on Marine Information Overlays (USA)
80
Background
Marine Information Overlays 2 (MIOs)
consist of supplementary information to
be used with an Electronic Chart Display
and Information System (ECDIS) that
are not Electronic Navigational Chart
(ENC) objects or specified navigational
elements or parameters. Supplementary
means additional, non-mandatory
information not already covered by existing
International Maritime Organization
(IMO), International Hydrographic Organization
(IHO), and International Electrotechnical
Commission (IEC) standards or
specifications. Current examples of MIOs
include ice coverage, tide/water level,
current flow, meteorological, oceanographic,
and marine habitats. Depending
on the navigational situation or current
task-at-hand, the provision and use of
MIOs (e.g., ice coverage, weather conditions,
etc.) can be crucial in terms of
improving both the safety and efficiency
of maritime navigation, as well as ensuring
the protection of the marine environment
(e.g., coral reef habitats).
As defined in the IMO Performance
Standards for ECDIS, an "Electronic Navigational
Chart (ENC) means the database,
standardized as to content, structure and
format, issued for use with ECDIS on the
authority of government authorized hydrographic
offices. The ENC contains all the
chart information necessary for safe navigation
and may contain supplementary
information in addition to that contained
in the paper chart (e.g. sailing directions)
which may be considered necessary for
safe navigation." In terms of being "supplementary
information", MIOs are not
contained within nor are they an integral
part of an ENC. Rather, MIOs are separate,
supplementary information that are
displayed in conjunction with the overall
System ENC 3 (SENC). This is similar
in concept to adding radar and AIS
information to an ECDIS display, and is
covered in the IMO ECDIS Performance
Standards, "Radar information or other
navigational information may be added
to the ECDIS display. However, it should
not degrade the SENC information, and
should be clearly distinguishable from the
SENC information".
The IMO Performance Standards for
ECDIS require chart data to conform to
IHO S-57 data standards, and that IHO
colours and symbols be used to represent
the System ENC (SENC) information.
While the current edition of IHO
S-57 (Edition 3.1) contains an ENC Product
Specification, it does not specify the
content or format for supplemental information
(e.g., MIOs). Similarly, neither the
current IHO Colours and Symbols Specifications
for ECDIS (IHO S-52, Appendix
2) nor IEC Publication 61174 (ECDIS
- Operational and Performance Requirements,
Method of Testing and Required
Test Results) specify how any supplemental
information should be displayed.

INTERNATIONAL HYDROGRAPHIC REVIEW
ENC Product Specification
As defined in IHO S-57 (Edition 3.1), Appendix B.1,
the ENC Product Specification is:
The set of specifications intended to enable Hydrographic
Offices to produce a consistent ENC, and
manufacturers to use that data efficiently in an
ECDIS the IMO Performance Standards for ECDIS. An
ENC must be produced in accordance with the rules
defined in this Specification and must be encoded using
the rules described in Appendix B1, Annex A "Use
of the Object Catalogue for ENC."
In an effort to insure the consistent and uniform
production of ENC data, IHO S-57 (Ed. 3.1) and associated
ENC Product Specification have been "frozen"
by IHO since November 2000. However, during
2006, IHO made some changes/extensions to IHO
S-57 to deal with an IMO requirement to include
the designation of Particularly Sensitive Sea Areas
(PSSA) and Environmentally Sensitive Sea Areas
(ESSA) on a paper nautical charts and ENCs 4 .
Since a chart-related MIO is intended to be used in
ECDIS or ECS in conjunction with an existing ENC,
it will conform - as much as practicable - to the
ENC Product Specification. This includes such criteria
as navigational purpose (compilation scale), cell
boundary, topology, feature object identifiers, meta
objects, mandatory (required) and supplemental attributes,
horizontal or vertical datums, units, etc.
Unlike for ENC data, MIOs are not restricted in the
use of time-varying objects that contain dynamic/
temporal information (tides, water levels, current
flow, wind, waves, etc.). However, there are some
specific requirements pertaining to the production
of consistent and uniform MIO data that would be
best met by developing a general content specification
for all MIOs.
General Content Specification for MIOs
Since there many types of types of MIOs that can
be produced, there is a benefit of having them conform
to a general content specification. Although it
will comply as much as possible with the S-57 ENC
Product Specification, it will also follow the strategy
used by NATO to produce Additional Military Layers
(AMLs). Similar to MIOs, AMLs are supplementary
layers of information that are used in conjunction
with a NATO Warship ECDIS that uses IHO S-57 ENC
data.
More specifically, the development of a General
Content Specification for MIOs was similar to the
approach recently taken by the UK Hydrographic
Office in its recent consolidation of the various
Product Specifications previously developed for
specific types of AMLs. In particular, the NATO AML
for Routes Areas and Limits (RAL) appears to be
most applicable to MIOs. The main benefit of this
approach is that ENC Software manufacturers (e.g.,
CARIS, SevenCs, and dKart) will not have to develop
new software tools to deal with MIOs. What is currently
used to produce AMLs will only require minor
modification to produce MIOs. Further, ECDIS and
ECS manufacturers will be able to interpret and display
MIOs similar to what is done currently for AML
and ENC data.
A General Content Specification for MIOs (Edition
1.0), dated 24 May 2007 was finalized at the 4th
Meeting of HGMIO in Durham, New Hampshire, USA
on 22-23 May 2007.
Development of a MIO Encoding Guide
Although IHO S-57 provides specific guidance (rules)
on how ENC data is to be encoded (i.e., the ENC Product
Specification), additional information is needed
related to encoding other S-57 objects, attributes
and attribute values that are currently contained in
the IHO S-57 Object Catalogue. This will also be the
case for newly-created S-57 objects, attributes and
attribute values that may be registered on the Open
ECDIS Forum (OEF) or on the future IHO Registry
for IHO S-100 standard 5 . Based on the strategy
that was adopted by the Inland ENC Harmonization
Group (IEHG) in order to produce new objects/attributes
for real-world inland/river requirements not
contained in the S-57 Object Catalogue, an "Inland
ENC Encoding Guide" was produced.
For all object classes, attributes, and attribute values
that are required to produce an Inland ENC, the
"Inland ENC Encoding Guide":
1 Provides a basis for its creation
2 Describes its relationship to the real-world entity
3 Provides criteria for its proper use
4 Gives specific encoding examples
A similar approach will be undertaken to develop a
"MIO Encoding Guide."
The Development of a "MIO Encoding Guide" was
discussed at the 4th HGMIO Meeting. Currently, a
prototype version is being produced in conjunction
with a MIO Testbed Project dealing with coral reef
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INTERNATIONAL HYDROGRAPHIC REVIEW
habitats and Marine Protected Areas (MPAs) in the
Florida Keys National Marine Sanctuary. The primary
purpose of having an "MIO Encoding Guide" is to
provide detailed guidance on what is required to
produce a specific type of MIO in a consistent and
uniform manner - anywhere in the world. An additional
benefit of using an "Encoding Guide" - both
for Inland ENCs and MIOs - is that it will be a living
document that can accommodate change. This is
not the case for the current IHO S-57 ENC Product
Specification which is "frozen".
Framework for International MIO Specifications
A document on Recommended Procedures for the
Development of Marine Information Overlays (MIOs)
was initially developed in December 2004, and approved
at the 17th IHO CHRIS Meeting in September
2005. An updated version of these procedures (Edition
1.1, 24 May 2007) that contains minor wording
changes (e.g., overlays) was prepared following the
HGMI04 meeting.
The procedures for MIO development provide guidance
on:
How a "competent organization" should identify
MIO-related requirements
Information content for a MIO category
Development of new S-57 objects and attributes
Appropriate colours and symbols, based on IHO
S-52
Test and evaluation
Production/dissemination of MIO data
Potential regulatory requirements on proper use
The overall framework for internationally-accepted
MIO specifications includes several components:
IHO S-57 Edition 3.1/3.1.1, where applicable.
Development of a harmonized MIO Encoding
Guide
A central register for MIO object classes, attributes
and attribute values.
Use of the Open ECDIS Forum (www.openecdis.
org) as a means for communication and publication.
Align with the future edition of IHO S-100.
Alignment with Future IHO-100
Work is ongoing within IHO to replace the current
Transfer Standard for Digital Hydrographic Data (S-
57) with a new IHO Geospatial Standard for Hydrographic
Data (S-100). Since IHO S-57 3.0/3.1 is
used almost exclusively for encoding Electronic Nav-
igational Charts (ENCs), there is a need for a more
robust standard to deal with changing requirements,
customers and technology for hydrographic data.
The primary goal for S-100 is to support a greater
variety of hydrographic-related digital data sources,
products, and customers. This includes matrix and
raster data, 3-D and time-varying data (x, y, z, and
time), and new applications that go beyond the
scope of traditional hydrography (e.g., high-density
bathymetry, seafloor classification, marine GIS). It
will also enable the use of web-based services for
acquiring, processing, analyzing, accessing, and
presenting data. S-100 will not be an incremental
revision of S-57 3.1. S-100 will be an entirely new
standard that includes both additional content and
support of a new data exchange formats.
Due to the worldwide prominence of ISO standards,
IHO S-100 will be based on the ISO suite of standards.
However, alignment with the ISO 19100 series
of geographic standards will require a re-structuring
of IHO S-57. More specifically, this requires a
new framework, and the use of new/revised terms
used to describe the components of S-100.
IHO plans to release S-100 in late 2007/early
2008. However, S-57 3.1/3.1.1 will continue to be
used for many years to come - even after S-100
has been released. Since current ECDIS equipment
are required to use ENC data conforming to the S-
57 ENC Product Specification, MIOs will continue to
be produced based S-57 3.1/3.1.1. Following the
adoption of any future ENC Product Specification
based on S-100, a determination will be made on
how to produce MIOs suitable for use with both S-
57 and S-100 based ENCs.
References
1 A HGMIO Information Paper agreed to at the 4th HGMIO
Meeting, Durham, NH, USA 22-23 May 2007.
2 Name change of Marine Information "Overlays" was agreed at
the 4th HGMIO Meeting, 22-23 May 2007.
3 System ENC (SENC) is the data held in the ECDIS system
resulting from the transformation of the ENC for appropriate
use.
4 "Enhancements Required to Encode S-57 3.1.1 ENC Data"
(S-57 Supplement No.l), IHB, Monaco, January 2007. [www.
iho.shom.fr]
5 IHO Geospatial Standard for Hydrographic Data (IHO S-100).
An information paper about IHO S-100 is posted on the IHO
website: www.iho.shom.fr
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
Note
Spatial Solution To Determine A Trigonometric Point
Of High Precision
By S.Ackermann, Università degli studi di Napoli "Parthenope" and A.Vassallo,
Istituto Idrografico della Marina (Italy)
The three "Spheres Of Position" whose radius are the three
Geodimetrical distances or "Slope Distance" of GPS
A, B, C are three trigonometric points of known coordinates, and d 1 d 2 , d 3 , are the
respective distances measured from the unknown point O; the equations of the
three spheres, with their centers at A, B, C and their radii d 1 d 2 , d 3 , are given by
(1)
where the rectangular coordinates of the three known points are
Developing (1) with the statements
they can be written
(2)
In Figure 1, we show one of the many configurations of the system of trigonometric
points involved in the solution of the system (2); in Figure 2 we have a plain (plain
of the sheet) passing for the unknown station and normal to its vertical. The three
circumferences with radii r 1 r 2 , r 3, are respectively the traces on plain of the three
spheres of radii d 1 d 2 , d 3, and the points A', B', C are the traces of the respective
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INTERNATIONAL HYDROGRAPHIC REVIEW
Figure 1 Figure 2
verticals of points A, B, C on the same plane.
The continuous line circumferences are relative to the distances without errors, while the dotted ones are
relative to the measured distances with a systematic error .
The next step is to determine the two radical planes (intersection of the pair of spheres AB and the pair BC)
which on plain determine the traces p 1 and p 2
(3)
the intersection of the radical line (3) (its trace on plain
gives two points, one of them is the sought point.
is the point T) with one of the three spheres above
(4)
The equations' system (4) formalizes the intersection of the line (3) with one of the three spheres (see
equations (2)) to obtain the station's coordinates; actually it is better to put, in the system (4), the equation
of the sphere with the minor radius to have an intersection less sensitive to the errors in the measured
distances (radii of the spheres).
We now state
when replaced in (4) we get:
(5)
Multiplying the first equation of (5) by N 2 and the second by N 1 and solving the system with respect to X:
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INTERNATIONAL HYDROGRAPHIC REVIEW
stating
(6a)
it can be written:
Likewise, multiplying now the first equation of (5) by M 2 and the second by M 1 and solving the system with
respect to Y:
stating
(7a)
it follows
At this point, replacing (6b) and (7b) in the third equation of the system (5), we get an equation which is
function of Z
developing the (8), we get:
and again
With the statements
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INTERNATIONAL HYDROGRAPHIC REVIEW
we can write the quadratic equation
(10) aZ 2 -2bZ + c = 0 .
The two solutions of equation (10) are
that, placed in (6b) and in (7b), will give the rectangular coordinates of two stations of which one is the
sought point.
X 01 , Y 01 , Z 01 and X 02 , Y 02 , Z 02
To demonstrate the validity of the method, let us proceed with a rigorous, hence with no errors, numerical
example, with the known station used only to calculate the three distances exactly.
Let the coordinates ED50 of the three points observed be
and the station's coordinates
transforming all of them into rectangular coordinates, we get
With the foregoing symbolism, we calculate the exact distances and we get
OA = d 1 =57923,54634 m
OB = d 2 = 43893,46675 m
OC = d 3 =47053,10306 m
R 1 = 6370988,84592
R 2 =6370227,67119
R 3 = 6370103,19754 ;
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INTERNATIONAL HYDROGRAPHIC REVIEW
using the previous values in the equations' system (4) we get:
following the declared (6a) and (7a) we get
then, replacing these last ones in (9) we get:
a = 2,4055363758
b = 9884542,9722487
c = 40616383922090
and consequently we have the two solutions
Z 01 =4109450,31880
Z 02 =4108710,97906
which, replaced in (6b) and (7b), give the rectangular coordinates of the two points, of which one is just the
station
X 01 = 4700444,85009
Y 01 =1261944,54954
Z 01 =4109450,31880
X 02 =4699591,03802
Y 02 =1261746,29764
Z 02 = 4108710,97906
To transform rectangular coordinates into geodetic coordinates, we use one of our formulary of rapid convergence
and we get the coordinates of the two stations, one of which is the real one.
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INTERNATIONAL HYDROGRAPHIC REVIEW
In the case at hand, the negative elevation excludes the point 0 2 ; alternatively the two elevations could be
both positive, but a rough estimate of the elevation of the zone (contour line, for instance) will be sufficient
to choose the right point.
Repeating the procedure by introducing a systematic error of 1cm in the measured distances, we get
OA = d 1 = 57923,55634 m
OB = d 2 = 43893,47675 m
OC = d 3 =47053,11306 m
R 1 = 6370988,84592
R 2 =6370227,67119
R 3 = 6370103,19754
and using the new values in the equations' system (4), we get
then, following the statements (6a) and (7a), we get
The (9), when these last values are replaced in, take to
a = 2,4055363758
b = 9884542,97354298
c = 40616383932099,9
so that the coordinates of the two points are:
X 01 = 4700445,26129 X 02 = 4699590,62798
Y 01 =1261944,64039 and Y 02 =1261746,19780
Z 01 =4109450,67491 Z 02 =4108710,62403 ;
the same, in geodetic coordinates, are:
and
( 1 ) - The bold numbers of the values of the last coordinates enhance the consequence of the introduced systematic error.
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INTERNATIONAL HYDROGRAPHY REVIEW
Comparing the coordinates of the two points, the height of the second reveals that the point O 1 is the good
one. As we can see, the geodetic accuracy in planimetry is kept, while the elevation is quite different from
the real elevation of the station. Anyway, that difference is acceptable in a trigonometric levelling.
In conclusion, with a systematic error on the distance of about 1 cm, we can state that it does not affect the
geodetic accuracy of the point; besides, the closer the distance values become, the more the influence of the
error on the sought point lessens.
Bibliography
- Birardi, C. (1967). Corso di Geodesia, Topografia e fotogrammetria, Parte II, Geodesia teorica. Istituto
Geografico Militare, Firenze.
- Levallois, J. J. (1969). Gèodèsie Genèrate. Edition Eyrolles, Paris V.
- Bomford, G. (1965). Geodesy. Oxford 17 T the Clarendon Press.
- Ewing C. E., Mitchell M. N. (1970). Introduction to Geodesy. American Elsevier, Publ. Com. Inc., New York.
S. Ackermann
Dipartimento di Scienze Applicate, Sezione di Geodesia, Topografia e Idrografia
Università degli studi di Napoli "Parthenope", Centra Direzionale, Isola C4
80143 Napoli
Italy
Email: sebastiano.ackermann@uniparthenope.it
A.Vassallo
Istituto Idrografico della Marina
Genova
Italy
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INTERNATIONAL HYDROGRAPHIC REVIEW Vol. 8, No. 2 November 2007
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