paulflexes saidYeah I watch too many documentaries.
Well as a pilot you must deal with magnetic anomalies all the time. In the Army I merely had to deal with land navigation via magnetic compass, before the days of GPS.
Our biggest problem was that Magnetic North is in northern Canada, not at the North Pole. Hence our military grid maps, which are drawn to polar north, have a magnetic declination, or difference, between these 2 norths. And Magnetic North does drift a little over time, so you have to readjust your calculation periodically.
For instance, your compass may be pointing to 0º N, but that's not true north. If you want to travel to true north you must consult your map legend for the degrees of magnetic declination, and alter your compass heading by the same number of degrees.