128 UNDERWATERSURVEY
record positions in three dimensions and it is very easy
for divers to take the measurements. In addition, standard
survey processing techniques can be used to compute the
positions of the features and also show how good those
positions are. The main drawback with this technique, how-
ever, is that it requires a computer program to compute
point-positions efficiently from the measurements.
The problem of positioning a point in three dimen-
sions can be simplified by separating the horizontal position
from the height or depth. Most underwater sites are three-
dimensional and the tools available to survey them are
usually limited. A tape-measure can be used to measure
the distance between points so that those points can be
drawn on a site-plan. But what if there is a big difference
in height between the two points? In such cases, the distance
measured is a slant distance and not a horizontal distance
that can be drawn on a piece of paper. It is straightforward
but time-consuming to correct each slant distance to a
horizontal distance, so a technique that uses slant distances
is preferable. A very high degree of accuracy can be
achieved using 3-D trilateration, as this is the most accu-
rate and reliable technique that uses tape-measures. A good
survey point position computed using this method can
be as accurate as 20 mm (95%), though this is dependent
on the accuracy of the control point network.
Three-dimensional trilateration requires a network of
survey control points to be established around and inside
the site. This network of points is the framework on
which the whole site-record is built. These control points
are used for the same purpose as the concrete triangula-
tion pillars (trig points/trig stations) found on the tops
of hills and mountains. Trig points are used for provid-
ing survey control across countries and control points are
used to provide the same control across a site. The main
or primary control points should be established around
the outside of the site. These points should be placed where
they will not be removed if the site is excavated. If a net-
work of survey control points is not used, it can be difficult
to make the measurements fit together or to prove the accu-
racy of any survey. If the position accuracy specified in
the project design is not achieved in the control network- or, worse still, not even measured – then the subsequent
survey work can never achieve the required accuracy.
The first step in establishing the network is to plan the
positions of the control points based on the current
knowledge of the site. Planned positions for the control
points are often based on the results of a previous assess-
ment survey. Next, the positions of the control points
are calculated using distance measurements between
them, plus depths for each point. The calculated positions
Figure 14.11 A diver taking DSM measurements. (Drawing by Graham Scott)