Example of Magnetic Declination:

Hence, the horizontal angle made by magnetic meridian and true meridian at a place is termed as magnetic declination of that place as depicted in Figure. The magnetic declination could be East declination Figure (a) or West declination as in Figure(b) indicating whether magnetic North is toward East or towards West of true North.

(a) East Declination δE       (b) West Declination δW

Figure: Magnetic Declination

It may also be noted here that position of magnetic North may change even at a station due to several factors. Therefore, it might be essential to record the date of survey and time to obtain the true bearing of a survey line at a place. Astronomical observations are required to be taken to determine the direction of true North at a place and hence to obtain the true bearing of a survey line at that place. Magnetic bearing can be easily obtained for the same line by compass survey. The difference between true bearing and magnetic bearing of the survey line so obtained will give the magnetic declination at that place, that is

Magnetic Declination = (True Bearing - Magnetic Bearing)

The magnetic declination would be positive if magnetic meridian is towards east of true meridian and negative if magnetic meridian is westward of true meridian.