How To Calculate True Bearing From Magnetic Bearing - || Rules & Example|| - Civil Engineering

How To Calculate True Bearing From Magnetic Bearing -|| Rules & Example||

If the magnetic declination at a place at the time of observation is known, the true bearing of a line can be calculated from its magnetic bearing and vice-versa.

Determination of True Bearing From Magnetic Bearing Rules

The following rules are applied to determine the true bearing if the magnetic declination is known:

1. If Declination is East

• True Bearing(T.B) = [Magnetic Bearing(M.B) + Declination]

2. If Declination is West

• True Bearing = [Magnetic Bearing - Declination]

Calculate True Bearing From Magnetic Bearing- Example

Example:
The magnetic bearing of a line OP is 89°45՛. Determine the true bearing if -
a) The magnetic declination is 5°30՛ E
b) The magnetic declination is 4°15՛ W

Solution:
a) In fig. (i)
True Bearing = (Magnetic Bearing + Declination) = (89°45՛ + 5°30՛) = 95°15՛. Calculate True Bearing From Magnetic Bearing When Declination is 5°30՛ E Fig. (i)
b) In fig. (ii)
True Bearing = (Magnetic Bearing - Declination) = (89°45՛ - 4°15՛) = 85°30՛. Calculate True Bearing From Magnetic Bearing When Declination is 4°15՛ W Fig. (ii)