Geoscience Reference
In-Depth Information
Contact method for measuring the orientation of a
linear feature
Either a Brunton-type or a Silva-type compass-clinometer can
be used for the contact method (Figure 2.9, pp. 18-19). The
Brunton-type has the advantage that the azimuth can simply be
read off without having to rotate the compass dial while also
holding the notebook.
2
Sighting method for measuring the orientation of a
linear feature
A more accurate method of measuring a linear feature, and one
that is easier with only two hands, is the sighting method
(Figure 2.10). This can be done easily with a Brunton-type
compass-clinometer and the azimuth reading can also be done
with a Silva-type compass-clinometer although it is not as
accurate. The slope of a large surface or hillside can also be
measured in the same way as the plunge.
2.3.3 Triangulation: Determining location
using a compass
A further important use of the compass is to determine your
position by taking a compass direction (bearing) off at least
two, and preferably three, features that are marked on the
topographical map of the area - a process known as
triangulation. The most accurate triangulation will result from
using features that are between about 60° and 90° apart, but in
some cases this is not possible. The closer the features are to
you the more accurately you will be able to fi nd your position.
The types of features used are: the corner of a building, corner
of a tree plantation, road junction, river confl uence,
telecommunications mast, the top of a hill or other distinct
feature. Ensure that you take the azimuth and draw the line on
the map using the edge of the building or particular side of the
road (rather than the middle or general direction) because any
small variation will result in an inaccurate position. The point
at which the two or three lines cross determines your position.
If the lines do not cross, one or more of your azimuth readings
is poor; recheck your azimuth readings and/or pick another
feature to sight off. Alternatively take the midpoint of the
triangle (see Worked Example 10.2 and Figure 10.5).
If you know you are somewhere along a linear feature such as a
road just one azimuth sighting from a feature at a high angle to
the linear feature can be used to produce a single intersection.
However, an azimuth reading from a second feature will reduce
the possible error. The steps to determine your location on a
map for the two different types of compass commonly available
are described in Figures 2.11 and 2.12, pp. 22-24. The Silva-
type compass has three signifi cant advantages for this type of
