It is essential to study the site before starting any further
survey work and generating a sketch is a good way to do this.
For a radial survey a diver records the position of features
on a site using a distance and bearing back to a single control point. This
technique is very simple, quick and can be done by a single diver. The technique
is not accurate enough for pre-disturbance or excavation surveys but gives a
good enough idea of the site for an assessment.
|Offsets and Ties
Offsets and ties are used to position features relative to a
baseline running through the site. An offset measurement positions a feature
using a single measured distance at right angles to the baseline from a known
point, a tie uses two or more measurements from known points on the baseline to
position the feature.
Three-dimensional trilateration or Direct Survey Method
(DSM) uses distance and depth measurements to position features on a site. The
technique is similar to 2D trilateration with the exception that distances are
measured directly to features and the processing deals with any difference in
Drawing frames are used to record very small areas of the
site in detail. A frame is placed over the area to be recorded and the diver
draws visible features on a scale drawing or directly on to the drawing frame.
Close-range three-dimensional photogrammetry is a very
accurate way of recording complex structures. A number of photographs are taken
of the structure from different viewpoints and processed in a computer program.
The program then constructs a three-dimensional model of the structure to scale.
If a series of overlapping photographs are taken of a site
then they can be joined together to form a single large photograph. The
photographs can be of the seabed taken vertically downwards or of the side of a
structure. Since the tools to do this have become more widely available this
technique is being used more often. Sufficiently good results can be obtained
with a digital camera and the graphics processing software available found on
Like close-range photogrammetry and drawing frames, this
technique is effective for small areas but must be used in conjunction with a
control point network.
Acoustic positioning systems are widely used for survey work
in the offshore industry. These systems effectively replace the tape measures
and dive computers used for DSM with distances measured with sound pulses. A
diver fitted with suitable hardware can be positioned within an array of
acoustic beacons, on the surface the archaeologists can then see where the diver