88 POSITION-FIXING
Position-fixing using horizontal sextant angles can be
accurate to within 1 m radius in ideal conditions, but
practice is required to take observations from a moving
boat. In the commercial world, sextant surveying is no
longer used, having been replaced by cheap, versatile
electronic position-fixing devices such as hand-held GPS
units (see below).
Compass Bearings
A compass bearing could be used as a position-fixing device
in the context of an archaeological survey but there are
preferred alternatives. However, awareness of how a
compass can be used is important because it adds to a gen-
eral understanding of position-fixing.
To plot a position, a charted feature is aligned with the
sights on the bearing compass and a reading taken.
Bearings should be taken on at least two (but preferably
three) separate features and, ideally, with a difference
in angle between them of approximately 60 degrees. If
two bearings are plotted on a chart, either by using
the compass-rose printed on the chart or by physically
measuring with a protractor from magnetic north, the two
lines should intersect at a point coinciding with where
the readings were taken. A third bearing will act as an
indication of accuracy and should pass through the
existing intersection, creating what is often referred to as
a ‘cocked hat’ (figure 11.7).
A prismatic or hand bearing-compass, or binoculars
with a built-in compass, may be used from boat or shore.
These traditional hand bearing-compasses employ a com-
pass card rotating in a liquid, but hand-held electronic
fluxgate compasses are now available which give a digital
readout. These have a greater potential accuracy but the
models so far encountered only give a bearing to within
1 degree.
The simple procedure of taking a fix requires practice
to achieve consistent results in a moving boat, particu-
larly with conventional magnetic compasses, as the com-
pass card is normally moving continuously in response to
the movement of the vessel. The major drawback with all
magnetic compasses is their susceptibility to magnetic inter-
ference from electronic equipment, iron and steel. Great
care must be taken to ensure the bearing-compass is not
deviating. Unlike a ship’s compass, it is not normally
fixed in one position on the vessel. This means that itsFigure 11.5 Sextant angles can be scribed on plastic drafting film to within about 20 minutes of arc (one-third of a
degree). This can be sufficiently precise to plot a position on a large-scale chart. (Based on original artwork by Ben Ferrari)