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A compass error caused by magnetic materials on the boat. If your compass reads too high on a particular heading, the deviation is called "west," if too low, it is called "east."

Generally we can say that a compass with deviation on one heading must have deviation on other headings, but we don't know what they are... with the exception that on non-steel vessels, the deviation on one True heading will usually be equal and opposite on the reciprocal heading, thus giving us a way to find deviation without a chart or knowledge of local variation. See our text Inland and Coastal Navigation.

Official definition:

n. 1. The angle between the magnetic meridian and the axis of a compass card, expressed in degrees east or west to indicate the direction in which the northern end of the compass card is offset from magnetic north. Deviation is caused by disturbing magnetic influences in the immediate vicinity of the compass.

Semicircular deviation changes sign (E or W) approximately each 180 change of heading;

quadrantal deviation changes sign approximately each 90 change of heading; constant deviation is the same on any heading.

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