- Tue Feb 28, 2017 9:27 am
#193648
The magnetometer points along the line of magnetic flux toward magnetic north. Which is not necessarily pointing to the earth's axis of rotation (true north). Only when polar north happens to be between you and the magnetic pole (or behind it) can you count on that direction. And then only in some years as magnetic north wanders around the poles over the years, albeit slowly. This needs to be accounted for by the person reading (maritime) maps, since it is position dependent. All part of the basics of marine navigation.
Aside from that those magnetic flux lines are dependent on the presence of iron or ferromagnetic material in the immediate surroundings of the compas/magnetometer. So steel or iron components in the boat, electromagnetic fields from energized coils, or mountainous regions or soil with elevated iron content in the vicinity of the boat could distort your sense of magnetic north. Before you venture out on a voyage based on the compass it's behavior when pointing in all directions should be known or corrected. (=calibrated, including roll, tilt and yaw)
GPS cannot tell where (true) north is if your position and path wanders all over the place. Only when you follow a straight path or it has a history of distant points where you have been can it tell how far off from (true) north you were on average. A GPS has no sense of direction at the point where it is. It needs to know where it was, plotting a line through recent coordinates.