How To Calculate True Bearing From Magnetic Bearing

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.

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՛.  

b) In fig. (ii) True Bearing = (Magnetic Bearing – Declination) = (89°45՛ – 4°15՛) = 85°30՛.