Compass deviation

Compass deviation is the angle between magnetic north and compass north caused by the ship’s own magnetism and ferrous structure acting on the compass card. It is named east or west and changes with the ship’s heading, which is why it is tabulated against heading in a deviation card. Deviation is reduced by adjusting the binnacle correctors: Flinders bar, quadrantal spheres, and the fore-and-aft and athwartship permanent magnets. Residual deviation is found by swinging the ship and comparing compass bearings with known true or magnetic bearings. Compass error equals variation plus deviation, both signed easterly or westerly.