Trip report regarding remote sensing activities at ... - Up To - SOPAC

TRIP REPORT REGARDING REMOTE SENSING ACTIVITIES AT THE STATION
POLYNESIENNE DE TELEDETECTION (SPT), DURING THE SURVEY IN PAPEETE,
TAHITI
INTRODUCTION
10 January - 15 February
This trip was implemented to meet the FIJI's 1989 work list and
the recommendations of the 17th CCOP/SOPAC session in Suva. FIJI.
Programme FJ.19 : "Baseline studies of inshore and nearshore areas in
Fiji for coastal development programmes".
The processing equipment at our disposal at the Station
Polynesienne de Teledetection (SPT) in Papeete is recorded in
another SOPAC report (Ref. 1). SPT is a French/Polynesian
(IFREMER/Territory of French Polynesia) organisation managed by L.
LOUBERSAC (IFREMER), assisted by two computer experts (Territory of
French Polynesia) and a French product developer.
The work undertaken was technically supported by the SPT experts. Two
satellite images on the BA River delta area were processed as part of
this work. Those two images were respectively : the SPOT image of the
NW of Viti Levu (shot in April 1988) and the LANSAT MSS image of the
same area (shot in November 1978). The SPOT image tape was offered to
SOPAC by IFREMER/SPT as a result of the SPOT pilot project policy. The
LANDSAT image diskette was offered to SOPAC by personal contact
(KUCHLER, D.) from Australia.
OBJECTIVE
The goal of the survey was dual :
- to test the possibilities of the satellite images,
in providing and monitoring useful information in submerged
delta area, and compare information from both
satellites;
-__ to obtain a good bathymetric process in a clear
lagoon environment, from the Aitutaki SPOT image.
This work was implemented ,together with the SPT experts, respectively
by Pascal COLLOTTE (first objective) and Olivier LEMAIRE (second
objective). The second objective is dealt with in a separate report
(Ref.l), it will not be treated here.
1

METHOD
1. Study of the existing documents
All mapping information relevant to the BA River area
was studied, especially the following :
_- the topographic map, Viti Levu, sheet 1 (1961),
_- the geologic map, Viti Levu, sheet 1 (1983).
_- the draft of the bathymetric map of the Sa River
Delta area (Ref. 2),
_ the positive film of the SPOT image
(1988),
-_ the paper print of the LANDSAT MSS image (1978).
For geographic incompatibility, the existing bathymetric
chart was not taken into account.
2. Processing the raw data (tape)
The satellite images are recorded on a Computer
Compatible Tape (CCT).
All processes used during the survey were obtained using the
SPT in-house image processing software named GRINGO.
The following method description takes the example of SPOT
data. Three channels are taken into account to load the
three SPOT channels :
-_ the infra-red band (XS3),
- the red band (XS2),
-_ the green band (XS1).
3. Decoding and extraction
Decoding the CCT is done in two stages, because the control monitor
can only manage a certain amount of data at a time. Therefore, we have
to select a restricted area. The control monitor used at SPT 1s a userprogrammable
image and graphic display; it is designed to off-load
display and interactive processing task from a central image
processing system. The maximum capability of the IVAS monitor is 1024 x
1024 pixels (elementary image unit). The size of the pixel is different
from one satellite to another. SPOT XS mode (colour) pixel size is 400
sq metres and LANDSAT MSS pixel size is 4800 sq metres.
In a first stage, the geometric coordinates of the selected
zone are calculated from the film or print of the image.
2

In a second stage, the calculated zone is extracted from the CCT
and loaded to the computer disk memory.
This operation must be done in interactive mode to control in
the mean time the area being extracted. This extracted image is
then loaded in three of the IVAS working channels :
-_ the infra-red satellite band in the red channel, _
- the red satellite band in the green channel,
-_ the green satellite band in the blue channel.
This choice permits the most realistic colours on the
products.
4. Image processing (see synopsis in appendix I)
The following operations are conducted on the area
extracted off the full image (1024 x 1024 pixels).
4.1 Histogrammes calculation
For each channel, the software allows the user to calculate
the histogrammes (luminance versus number of pixels). This
operation, done in interactive mode. It permits a direct upgrade
of the image on the screen by using linear functions. At this
stage the image is easily readable. This allows for a pre-study
of the image, giving an idea of the best type of processing to
use, as well as a precise idea of the importance of the density
of information in each channel.
4.2 Geographic positioning of the image
The existing maps are in UTM protection, at a scale of 1/50000.
Then, it is important to position the image in the same grid for
consistency. It is also important to do this rectification at
this stage to avoid the geographic rectification of each
intermediate result during the processing. For SPOT data, it
consisted mainly of a rotation of 6 degrees east. The image will
be rectified point by point and compared to contour lines
digitised from the most recent map in choosing fix-marks from
it. A polygon of deformation is calculated via the software, and
applied to the image by interpolation from the closest neighbour
pixels (Ref. 3).
3

4.3 Zoning of the image
While processing the image, it appeared that 4 different
zones can be defined off it :
- the reef area,
- the closed lagoon area covering the submerged
delta,
- the mangrove area,
- the land area.
These zones of geological interest are much different, they
will be processed separately. To separate the zones, we
graphically contour them in interactive mode, using the forth
IVAS channel. Then, a mask is created overlaying each zone,
from which the areas are calculated.
5. Processing of the rectified and zoned image
Each zone is processed separately because of its
reflectance and of its distinct geologic interest.
5.1 Calculation of the transfer function
The choice of the mathematical function depends upon the type
of information we want to come up with. Therefore, we used the
following functions for processing the SPOT image of the Ba
River area :
- a linear upgrading function applied to areas,
in between limits or not, of the chosen zone;
- an exponential function applied inside the limits of
the histogramme peak, to upgrade preferentially the
high pixel luminance classes in the lagoon zone;
- a function of distribution of the pixel classes
(function of their luminance), which re-divides the
pixels in classes of equivalent number, called
equipopulation distribution.
More complex functions are also employed to determine
different ratings (see appendix II). The following ratings are
used to process the different zones.
- chromatic rating (Ref. 4), which is use to show the
turbidity of the water and the shallow lagoon bottom; -
vegetation rating (Ref. 5), for the mangrove zone;
- soil shining rating (Ref. 5), to differentiate bare soils
in the land and wetland zones.
4

5.2 Application of functions and ratings
The functions and ratings are applied through the mask of
the considered zones.
5.2/1 The reef zone
An equipopulation distribution is applied on the green and blue
channels. Only a linear function is applied to the red channel,
to smooth the artifact record of the wave breaking in the infrared.
5.2/2 The lagoon zone
A chromatic rating was applied to the entire zone. This allows
for differentiating the delta bottom from turbid waters. An
exponential function is applied on the submerged area of the
delta. This processing produces a monochrome image, which will
be loaded on each channel (R, G, B).
5.2/3 The mangrove zone
On this particular zone, at first a vegetation rating is applied
on the tidal flats to separate them from the rest of the zone.
After masking vegetation areas, an equipopulation distribution
is applied through the mask. For a more realistic appearance, it
was interesting to cross-load the red and the infra-red SPOT
channels respectively in the red and the green loading channels.
5.2/4 The land zone
An equipopulation is applied allover the zone.
6. Recomposition of the resulting image
Once they have been processed separately, the zones are
brought together to recompose the resulting image.
7. Compilation of the thematic map
The geographic grid, the culture and the legend are added
to the image. Because mixing between raster information (image)
and vector information (graphic) is not yet implemented, the
grid and the legend are added to the image through raster
transfer.
7.1 Grid
The grid is recalculated from the topographic map of the
area of interest and drawn using fix-marks.
7.2 Legend
The facies reference boxes are obtained in sampling windows
from the concerned zones of the image. The location of boxes and
text is calculated on the legend sheet before positioning.
5

RESULTS
Three maps were produced at the scale of 1/50000, : -
the Ba River Thematic Map (Viti Levu, FIJI), using the
SPOT image (April 1988);
_- the Ba River Thematic Map, using LANDSAT MSS data
(November 1978); the same processing as SPOT data
processing was applied to allow for a comparison of the
maps to monitor the changes within 10 years time;
-_ the comparison map between SPOT data and LANDSAT data in
the submerged part of the Ba River Delta.
Four zones are identified :
- the reef zone,
- the submerged river delta zone,
- the mangrove zone,
- the land area.
Various types of facies have been identified in each zones,
using only satellite information.
In the lagoon:
- a partition reef facies,
- two types of shallow marine reef,
- pinnacles,
- emerged surficial coral sands,
- submerged deltaic deposits.
In the wetland zone:
- tidal flat bare soils,
- a bushy type of mangrove,
- a shrubby type of mangrove.
Inland:
- the forest areas,
- a "bush" facies
- the inland bare soils,
- the sugar cane fields.
6

CONCLUSION
These three satellite products being geographically
rectified and reduced to a compatible scale, they are able to
be used with usual map format.
On the southern part of the Tavutha reef, a shrubby type of
mangrove growing on coral sand deposits is recognised.
The submerged delta appears very well as well as the main
channels of river and creeks and their distributaries.
Two types of mangroves are identified, one is pioneer (bushy
mangrove), the other one attests of an evolution. The tidal
flats are also identified.
The comparison between SPOT and LANDSAT MSS data shows the
evolution of the shrubby mangrove on the east part of the
wetland zone. But no big difference appears in the submerged
delta zone in 10 years time.
In the land area the bare soils appear very well, especially the
basins of the Ravi-ravi prawn farm. The forest areas and sugar
cane fields are also easily mappable
7

APPENDIX II
The following rating, have been used to process the
satellite images of the Ba River Delta area. They are based on
equations involving the wave length bands scanned by the
satellite sensors.
BANDS! SPOT ! LANDSAT"
! !
Green band! XS1 = 500/590 nm ! BAND 5 = 500/600 nm
Red band! XS2 = 610/ 690 nm! BAND 6 = 600/700 nm
I.R band! XS3 = 700/890 nm ! BAND 7 = 700/800 nm
1. CHROMATIC RATING
This algorithm is upgrading sediment information in shallow
water.
If A and B are the peak limits of the histogrammes of the 3
bands, the chromatic rating is equal to : rXS2/XS1+XS21,
between A, B.
2. VEGETATION RATING
This algorithm makes it easier to differentiate the
vegetation from the bare soils. It is equal to : rXS3-
XS2/XS3+XS21

APPENDIXES

REFERENCE
Ref. 1: LEMAIRE, O.
Report on the Aitutaki processing at the Station
Polynesienne de Teledetection, Tahiti. 10 January - 8
February 1989.
CCOP/SOPAC trip report (pending)
Ref. 2: GRINGO version 2, manuel utilisateur
Rapport interne IFREMER, 1987.
Ref. 3: LOUBERSAC, L.
Coastal zones inventory by high resolution satellites. Remote
sensing Processing Alpbach Summer School, ESA SP-205, pp 87-94.
Ref. 4: JUPP, D.L.B., HEGGEN, S.J., MAYO, K.K., KENDALL, S.W.,
BOLTON, J.R., and HARRISON,B.A.
The BRIAN hand book : an introduction to LANDSAT and BRIAN
system for users.
CSIRO Division of Water and Land Resources, Natural
Resources, Series N.3, 1985
Ref. 5: LOUBERSAC, L. and POPULUS, J.
The application of high resolution satellite data for coastal
management and planning in a Pacific coral island. Geocarto
International Vol.2, pp 17-31, 1986.