Geoscience Reference
In-Depth Information
window that provides a measure of the azimuth in degrees away
from north. The azimuth method for determining direction uses
a circle with the value increasing clockwise from north at 0° (=
360°). On the Silva-type the dial can be rotated to place the
needle at 0°. The azimuth reading for the direction in which the
sight at the end of the mirror is pointing can be read off using
the 'marker for azimuth reading (1)' (Figure 2.3d). Note that
because the azimuth scale is fi xed in the Brunton-type and the
needle moves relative to this the compass is numbered and
labelled anticlockwise. The azimuth for the direction in which
the long sight on the Brunton-type is pointing (Figure 2.3b) is
the reading at the north end of the compass needle. Compass
directions from north can either be reported approximately, e.g.
northwest, east, etc., or to the nearest degree. The Brunton-type
compass also has a built-in locking pin for the magnetic needle
to temporarily hold the needle in place when a reading is taken
(Figure 2.3c).
2
BEWARE! Compasses can be
affected by rocks containing
magnetic minerals (e.g.
serpentinite, gabbro), iron
objects (gates, hammers,
cars), and wires with electric
currents passing along them
(e.g. power lines). Always
check odd readings!
The design and working mechanism of the clinometer part of
compass-clinometers varies between the different makes and
models. However, the principle of the clinometer is exactly the
same. On both types of compass there is a scale on the inner
part of the compass window to measure the magnitude of the
angle between the needle and the horizontal (clinometer scale;
Figure 2.3b, d and e). To use the clinometer part, the
instrument needs to be held with the compass window vertical
and the long edge at the same angle as the dipping surface. In
the case of the Brunton-type the long edge adjacent to the east
on the azimuth scale needs to be at the base because of the way
the clinometer scale is orientated. The Brunton-type compass
has a clinometer arm, the position of which can be adjusted
using the lever on the back of the device (Figure 2.3c). When it
is correctly adjusted for a particular dip angle the bubble in the
long level should be in the centre. In contrast, the Silva-type
has a clinometer needle that fl oats free and vertically
downwards when the device is held on its edge vertically. The
clinometer needle will hold its position if the instrument is
carefully tilted about 20° from the vertical to the horizontal. In
order to measure the dip on the Silva-type the compass dial
needs to be set so that (i) the 'marker for azimuth reading (1)'
(Figure 2.3d) is at 90° or 270° and (ii) the long edge of the
compass-clinometer is orientated so that the clinometer scale is
at the bottom where the clinometer needle is located. It may
help to think of the clinometer as a protractor within the
compass housing with a plumb line (the needle) indicating the
magnitude of the angle relative to the horizontal. To test how
your model works try holding the compass-clinometer as if it
was on a horizontal plane and then increasing the angle to 45°
and then to 90°. The operation of both types of compass-
clinometer for specifi c applications is explained and illustrated
later in this section.
