deviation and variation. Each of these is a disruption in the magnetic field that can affect a compass' ability to accurately ...

Hence, only when a compass is roughly on a prolongation ... but that from year to year the variation varies because the magnetic pole moves. No one knows why and no one knows precisely how much.

This is how the British Ordnance Survey publish the magnetic offset in the margin of their printed maps. [Fair use] Every former Scout will have learned how to take a compass bearing and relate it ...