Archaeology Underwater: The NAS Guide to Principles and Practice

UNDERWATERSURVEY 121

a second diver swim up and down the ‘offset’ tape to ensure
that it is not snagged.
In this case two distances are recorded – the ‘offset’
distance from the feature to the baseline and the ‘along’
distance from the zero end of the baseline to the start
of the offset (figure 14.5). The position accuracy of off-
set measurements is limited by the fact that it is difficult
to estimate correctly the right-angle with the baseline, and
it is therefore only suitable for use in good visibility and
when close to the baseline. Long tape baselines can move
sideways and do not provide a stable reference for the
offset measurement.

Ties/trilateration

This works by creating a triangle, taking two measurements
from a feature to two chosen points (control points)
on a baseline (figure 14.6). It is most accurate when the
triangle is as close as possible to equilateral. Although
this cannot be achieved precisely, the angle of the tapes
at the feature should be between 30 and 120 degrees:
this is known as the angle of cut.
As much as possible should be drawn up under water,
so that details can be added and errors can be identified
and fixed. Drawing up a plan-view on dry land, based on

either method, requires a scale ruler, a pair of compasses

and a set square. For both methods, select a suitable

scale based on the length of the baseline and the size of the

paper, draw the baseline to scale, and label the control

points. For offsets, mark the scaled measurements along

the baseline (from the appropriate control point) using

the scale ruler. A set square can then be used to mark the

distances out from the baseline (offsets) (figure 14.5(3)).

When drawing up ties measurements, set the compasses

to the correct measurement and draw an arc from one

control point and another arc of the right length from

the other control point. Where the two arcs cross is the

third point (figure 14.6). Once completed it will be clear

why angles which are too acute or too obtuse do not

produce an accurate result, as the point where the two lines

cross will not be as clear as when they cross at closer to

the ideal 60 degrees. Drawing up on a computer uses the

same principles.

The trilateration method does not need to be limited

to baselines. It can be carried out directly from any

two control points on a site. It can also be used to tie

in additional control points and is particularly useful

for recording isolated finds. For an area with tightly

packed archaeological material, a grid frame is usually

used (see below).

B

A

12 3

B

A

‘offset’

Tape

Zero end

‘along’ distance

Figure 14.5 1) Offset method; 2) a method of establishing a right-angle, 3) plotting offset results. (Based on original
artwork by Ben Ferrari)