Compass and bearings

A bearing gives the direction of a line as an angle from a reference direction, usually north. The whole circle bearing (WCB) runs clockwise from north through 0 to 360°. The reduced or quadrantal bearing gives the angle from north or south towards east or west, such as N 40° E or S 25° W. Converting between them is routine. The back bearing of a line, its direction read from the other end, differs by exactly 180°.

A prismatic or surveyor's compass measures magnetic bearings with a magnetised needle. Magnetic north differs from true (geographic) north by the magnetic declination, which varies with place and slowly with time, so bearings must be corrected for it when true directions are needed. Local attraction from steel structures, vehicles, power lines or iron-rich rock deflects the needle; it is detected when the forward and back bearings of a line do not differ by 180°, and corrected station by station from a line that is free of it.

Compass traversing is quick but of modest accuracy, suited to rough surveys, forests and reconnaissance. The interior angles of a closed compass traverse are computed from bearings and checked against (2n − 4) × 90°. Bearings combine with distances to compute latitudes and departures, the north and east components of each line, the basis of coordinate computation in all traverses, whatever instrument is used.