Magnetic deviation is defined as the difference between the north compass heading and the actual heading to true north. The difference arises because the earth is not a homogenous sphere but has a time- and space- varying magnetic core and field structure. The earth acts like a giant magnet with its source of magnetism not concentrated at specific magnetic poles but throughout a large volume within the earth. The geographic north pole and the numerous magnetic north pole are located in different positions. is a common misconception that a compass points towards magnetic north. This is not the case since a compass will point along the local magnetic field line. The magnetic variation at any point on the earth reflects this pointing variation.