How do we calculate where magnetic north is?
Rather than calculating the magnetic poles, they must be found by expedition. They are defined as the points on Earth’s surface where the magnetic field points vertically down, so you need to be on top of them to really know. It may be assumed that you could calculate its location by finding north with a compass from two different points, drawing lines on a map and finding the intersection of those lines. However, the Earth’s magnetic field is not evenly distributed or symmetrical, so the direction of north indicated by a compass in one area can differ greatly in another.
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Plus, take a look at:
What causes Earth’s magnetic field?
How Earth’s magnetic field works