Magnetic Declination:

The direction of magnetic meridian varies from place to place across the globe. Hence, the bearings taken with reference to magnetic meridian of the survey lines will not represent true relative angles between them. The errors can be negligible for smaller area surveys but will be quite significant for large surveys particularly geodetic and astronomical surveys. A more accurate method of denoting the bearing of a survey line will be to obtain the true bearing of the line. The difference between true bearing and magnetic bearing of a survey line at a survey station is called magnetic declination of that place.