Definition
When a vessel heels, the geometry of the vertical component of the Earth's field relative to the ship's own magnetic structure changes, altering the deviation. On a sailing yacht heeled 20 to 30 degrees the effect can amount to several degrees and reverses sign on the opposite tack, which is one reason a course held on one tack may not repeat on the other. It is largest at high latitudes where the vertical field component dominates.
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