90 POSITION-FIXING
intersection when plotted (figure 11.8). A third transit
will act as a check to see whether the observed features
actually are the ones charted. This system is simple and
potentially very accurate, particularly if the distance
between the two features in alignment is a large pro-
portion of the distance between the observer and the
nearest feature (figure 11.9). It is also inexpensive given
that all that is required is an appropriate map or chart,
and perhaps a camera to record the transits.
Although the technique is extremely useful there can
be problems. Archaeological sites under water and useful
charted features are rarely conveniently positioned rel-
ative to each other. A choice may exist between features
that do not quite line up and give an open transit
(features just apart from each other) or a closed transit
(one partially or totally hidden behind the other).
Often natural features have to be used and these can
be difficult to equate precisely with what is drawn on the
chart. For instance, the base of a cliff or the edge of an
island or rock can differ as a result of tide, weathering or
seasonal vegetation. In many instances the lack of suitable
charted features will dictate that uncharted features will
have to be used. Sometimes artificial transits, such as pairs
of surveyor’s ranging poles (figure 11.10) will have to be
placed in appropriate positions. As with uncharted fea-
tures, if these temporary alignments have to be used to
plot the position of a site, each will have to be separately
surveyed and marked on the appropriate chart or map.
Establishing beforehand which features will be available
for transits is not always practicable because they may not
be visible due to poor horizontal surface visibility, or because
they are masked by intervening landforms or vegetation.A
B CFigure 11.7 A large triangle of error, or ‘cocked hat’. The
smaller the triangle, the better the fix.
Figure 11.8 The use of coastal features as transit marks to establish the position of a site. (Based on original artwork
by Ben Ferrari; after Oldfield, 1993:195)