Question

The magnetic bearing of a line AB is S28°30’E. What is the true bearing of line AB if the magnetic declination is 7°30’ towards west ?
1. S36°E
2. N21°W
3. S21°E
4. N36°W

Answer 1

Correct Answer - Option 1 : S36°E

Concept:

Magnetic declination: At any place horizontal angle between true meridian and magnetic meridian is called magnetic declination.

For Eastern Declination:

α = β + θe or T.B = M.B + θe

Here, α = True bearing (T.B)

β = Magnetic bearing (M.B), and

θE = Eastern declination

For Western Declination:

α = β – θw or T.B = M.B – θw

Here, θw = western declination.

Calculation:

Magnetic bearing of a line AB = S28°30’E = 151°30’

Declination = 7°30’ West

∴ True bearing = 151°30’ - 7°30’ = 144° = S36°00’E