A variety of compasses are available at a considerable range of prices (see,
for example, a supplier's catalog). Of these, the Brunton compass (Fig. 2.3)
has the advantages of combining a precise and hand
level with a compass that can be used to measure bearings at either waist.
height or eye-height. The less expensive and more compact Silva Ranger
Compass is excellent for measuring strike and dip, or trend (bearing) and
plunge, but cannot be used as a precise sighting-clinometer or hand level.
The Silva compass also lacks a leveling system for sighting bearings and
thus is not as precise as the Brunton compass when traversing or making
locations by intersection (Section 6-3).
Brunton compasses, however, may be damaged when handled roughly,
and should therefore be checked before use to be sure that: (l) the compass
needle swings freely; (2) the hinges of the sighting arm and lid are firm; and
(3) the point Of the sighting arm touches the axial line of the mirror when the
arm and lid are turned together. The clinometer level, which may become
misaligned, can be checked by setting the vertical angle index at O and plac-
ing the compass on a smooth surface that has been leveled with a long car-
pen ter's level or an alidade (the bull's-eye level on the Brunton compass is not
precise enough). If the tube bubble does not move to center, the plastic (or
glass) cover must be removed and the level-mount rotated gradually until
the bubble is centered.
The compass must also be opened to clean or dry the needle bearing or to
level the magnetic needle by moving the small wire coil on it. For mapping
south of the equator, the coil must be removed and placed on the north-
seeking half of the needle.
The magnetic decllnatlon (the local difference between magnetic north
and true north) must be added or subtracted from all bearings taken with a
magnetic compass. The correction is made beforehand and automatically
for most compasses by turning the graduated compass circle the amount of
the local declination. The circle of the Brunton compass is turned by a screw