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@freemo @mjambon It's a little more complicated than that. Magnetic deviation can be caused by anything that produces or alters the magnetic field, including trace ferrite in a nearby hill, nearby power lines, or even the phone in your pocket. Then you also have other issues, such as magnetic variation (the difference between magnetic north and true north), and even additional counterintuitive errors caused by acceleration and turning.

For example, in every airplane cockpit, you'll see something like this (to compensate for local fields caused by electronics):