Archaeology Underwater: The NAS Guide to Principles and Practice

130 UNDERWATERSURVEY

to be extended as more of the site is discovered. The over-
all shape of the control point network should remain
circular or elliptical after it has been extended. Where the
site is large, secondary points can be placed within the
site so that long tape-measurements are avoided (figure
14.16). On some sites, these points can be permanent
and fixed to the sea-bed; however, this may not always
be possible.
Secondary points fixed to structure or large artefacts can
be used to span the gap between the two sides of the site

(figure 14.16). Long, thin networks or networks with a very

pointed shape should be avoided because the position error

will be large for the points furthest away from the main

body of the network (as shown in figure 14.17, which illus-

trates poor configurations of control points). The angle

between measurements to a point should be no smaller

than 45 degrees and no larger than 135 degrees for a good

control-point network shape.

It has been demonstrated that six distance measure-

ments are used to position four control points, with one

Circle ‘Outrigger’

Ellipse

Figure 14.15 Good control-point network shapes (3-D survey)

Secondary

point

Figure 14.16 Secondary points (3-D survey)