POSITION-FIXING 87
Methods of Position-Fixing
Horizontal sextant angles
The sextant is one of the most useful optical position-fixing
tools for coastal surveying. It is basically a protractor
and telescope linked by mirrors, which allow an angle
between two separate features to be measured from the
survey position (figure 11.4). Although superseded by
electronic devices, sextants are still used in certain cir-
cumstances because they are accurate and portable.
Traditionally, two angles between three charted objects
are used for position-fixing but, to increase confidence in
the fix, it is preferable to take a third angle involving a
fourth charted object, as this will give a check on the accu-
racy of the first two readings. Once a set of angles has been
taken, the position can be plotted on a chart using a num-
ber of different methods. The angles can be drawn on a
piece of translucent drafting film and then laid over the
chart and moved around until the lines pass through the
appropriate features, the intersection of the lines being
the plotted position (figure 11.5).
Alternatively the plot can be constructed geometrically
(figure 11.6), the simplest method being to draw a base-
line between the left-hand pair of features and draw a line
at an angle of 90 degrees, minus the measured angle, out
from each end. The intersection of these lines is the cen-
tre of a circle that has a radius equal to the distance between
the features and the centre. At any point on this circle the
angle between the two features will be constant. A sec-
ond circle must therefore be drawn, constructed in the same
way on a baseline drawn between another feature and one
of the pair already used. The intersection of these two
circles is the plotted position. It is possible to construct
a whole series of circles based on different angles, and
these horizontal sextant-angle charts can be very useful
if a lot of survey work is to be undertaken in the same
area using the same charted features.ACBFigure 11.4 Taking horizontal sextant angles. Angle A is measured between left-hand side of the flagpole and the left-
hand side of the church tower by lining them up in the split-viewfinder. Angle B is measured between the left-hand
side of the church tower and the left-hand side of the chimney. Angle C, measured between the flag pole and the left-
hand side of the chimney, should equal angle A plus angle B. (Based on original artwork by Ben Ferrari)