Geophysical Survey in Archaeological Field Evaluation - HELM

Part IV Practitioner’s Guide to
Good Practice
1 Application of techniques
1.1 The survey grid
Geophysical fieldwork relies on the presence
of an accurately plotted network of control
points extending across the area to be worked
on and this is usually referred to as the survey
grid. An internally accurate and correctly
georeferenced grid is crucial to all subsequent
survey and to the whole project outcome: close
attention to this fundamental stage of fieldwork
is therefore essential. Recent developments
involving mobile sensor platforms incorporating
real time global positioning system (GPS)
sensors mean that it is no longer always
necessary to establish a conventional grid of
fixed markers over the surface of the area to
be surveyed (see below, section 1.1.2). When
employing such technologies, survey teams
should recognise that a grid of control points
capable of accurately defining the boundaries
of the area surveyed is still required even if
not actually laid out with ground markers
before or during the geophysical survey.
However the survey grid is located, during
fieldwork a record should be made relative to
it of surface conditions and sources of modern
geophysical interference that might have a bearing
on subsequent interpretation of field data.
1.1.1 Conventional survey grids
Establishing and marking out the survey grid
are usually the responsibility of the project
manager, although this should be discussed
and clarified with the geophysical survey team
involved. The grid can be laid out by any
suitably qualified personnel with the agreement
of (and, if necessary, following the instructions
of) the geophysical surveyors. Considerations
of geophysical methodology or ground response
may well dictate a preferred grid alignment,
particularly when the alignment of linear features
is already known. In this regard, Gaffney and
Gater (2003, 85–8) provide a concise review
of the issues common to most archaeological
geophysical surveys.
Where deadlines are tight, a previously surveyed
grid will allow the incoming geophysical survey
team to concentrate their specialist time to
greater effect. Where more time is available,
they may otherwise wish to provide the
survey grid themselves. Whoever lays out the
grid, it is important that its internal accuracy
and measurements to fixed topographic points
are rigorously and independently checked.
Geophysical survey teams are advised always
to check the accuracy of previously surveyed
grids and to take independent measurements
for grid location. There can be no excuse
whatsoever for any subsequent mismatches
between different parts of a geophysical survey,
or other positional confusion. It is preferable
and convenient, but not essential, for the
geophysical survey grid to match exactly
the Ordnance Survey National Grid (see
for example English Heritage 2003) or a site
grid devised for other purposes, such as field
walking. The need to fit a survey into existing
boundaries may dictate the use of a different
grid, however. If more than one grid must be
used, accurate location of each will be critical
for the subsequent integration of results.
A unit of either 20m or 30m for the side of
each grid square is usual (although some survey
methodologies may use a different optimal
base survey unit), with grid intersections located
on the ground using wooden pegs or other
temporary markers, which must be non-magnetic
for magnetometer surveys. Because of the many
hazards involved, not least of which concern
the safety of people and animals, the choice
of markers and their duration in the ground
needs careful forethought as well as the
agreement of the landowner and/or tenant
(see also Part I, 7.1).
The grid must be laid out using currently
accepted conventional methods (eg Bettess
1992; Bowden 1999; Clark 1996).
For long grid lines, in excess of 100m, the use
of a theodolite, EDM total station or GPS is
advisable. For smaller grids, the use of an optical
square is acceptable (eg English Heritage 2002).
English Heritage (2003, 8–9) provides a useful
summary of the different types of measurement
accuracy associated with survey grids, defining
relative, map and absolute accuracy. Using any
of the aforementioned techniques it should be
possible to locate the grid control points on
the ground to a relative accuracy of ±0.1m.
GPS equipment is becoming increasingly available
and English Heritage (2003) addresses the
issues associated with its use for archaeological
survey, classifying the various types of GPS
system according to the positional accuracy
that can be achieved (navigation-grade, mapgrade
and survey-grade). Survey-grade GPS,
capable of absolute positional accuracy of ±0.1m
(either in real time or with post-processing),
is the only grade suitable for locating survey
grid control points. It should be noted that
the positional accuracy of existing base maps
may be lower, depending on how they were
originally created (see English Heritage 2003,
8–9). Bearing this in mind, it is advisable when
using GPS to locate the survey grid to measure
the positions of some fixed local landmarks or
boundaries recorded on the area base map
and not just record the temporary survey
grid points. Any discrepancies between GPS
positioning and local base mapping can then
be compensated for and it is also possible to
re-establish the grid independently relative to
the measured landmarks.
1.1.2 Interfacing with GPS
Recent developments in GPS technology mean
that it is now possible to interface geophysical
survey instruments directly to continuously
logging mobile (portable) GPS sensors, enabling
the position of each measurement to be
accurately located as it is taken (Fig 1). A
differential GPS system may be employed to
position measurements rapidly relative to a
field-based control station, which is subsequently
georeferenced to provide absolute accuracy
through post-survey processing. However, the
most recent real-time GPS systems can provide
immediate survey-grade absolute accuracy
by receiving broadcast signals from real-time
correction signal transmitters calculated from
a network of fixed control stations. With both
types of GPS system, it is possible to carry out
an accurately positioned geophysical survey
without first establishing a physical grid of
ground markers. It is important for the users
Fig 1 (above) The GEEP towed mobile sensor platform with built-in GPS (photograph courtesy of Ian Hill, University of Leicester).
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