116 UNDERWATERSURVEY
sternpost on an exposed site, or the depth of burial of a
site under the sea-bed (see chapter 17).
A topographical survey aims to record the shape and
characteristics of the sea-bed. The same principles are
used; what makes this different is the need to cover a
wide area. Recording the shape of the sea-bed usually
involves recording the depth or height of the sea-bed
at known positions. If the measurements are made at
regular intervals over the site, then a plan can be pro-
duced showing the depths as contours. Details con-
cerning the surface of the sediments and bedrock lying
under water should include information about relative
heights of features. Recording topography in detail is
time-consuming if it is to be done accurately. Sometimes,
more can be gained in a reasonable time by sketching
the site and adding spot-depths at known features.
Accurate wide-area topographic maps and 3-D models
are usually created using a multibeam echo-sounder
(see chapter 13). There are standard ways of describing
types of sea-bed and sediment. For further informa-
tion, readers are referred to Seasearch Observetion Form
Guldance Notes(www.seasearch – Seasearch Recording),
which provide detailed practical information on how
to record such information (see also table 14.1).An Initial Sketch
The first step in any survey is to create a sketch of the
site, as this will form the basis for any future work. A
good sketch can very quickly provide a large amount of
information about a site and this can be invaluable in theFigure 14.2 Translating the visual image into written record: the sea-bed (left) can be depicted by the use of symbols
as shown on the right. (Based on original artwork by Ben Ferrari)