Magnetic Bearing in Compass Surveying

Magnetic Bearing:

Magnetic bearing of a line is the horizontal angle which it makes with the magnetic meridian passing through one of the extremities of the line. A magnetic compass is used to measure it.

The common systems of notation of bearings are:

1. The whole circle bearing system(W.C.B) or Azimuthal system
2. The Quadrantal bearing (Q.B) system.

1) The whole circle bearing system (W.C.B) or Azimuthal system:

In this system, the bearing of a line is measured with magnetic north (or with south) in clockwise direction. The value of the bearing thus varies from 0 to 360 º.

Fig.18

Prismatic compass is graduated on this system. Referring to Fig.18,the W.C.B of AB is Ө₁,of AC is Ө₂,of AD is Ө₃ and of AF is Ө_4.
b) The Quadrantal bearing (Q.B) system: (Reduced bearing)

In this system, the bearing of a line is measured eastward or westward from north and south are used as reference meridians and the directions can be either clockwise or anticlockwise depending upon the position of the line. In this system, therefore, the quadrant, in which the line lies, will have to be mentioned. These bearings are observed by Surveyor’s compass.

Referring Fig.19, the Q.B of the line AB isα and is written as N α E, the bearing being measured with reference to North meridian (since it is nearer), towards East. The bearing of AC is β and in anticlockwise direction towards East. Similarly, the bearings of AD and AF are respectively S Ө W and NØ W

Fig.19