Deviation

Engines, steelwork, speakers, electric currents, tools, phones and other magnetic items can affect a compass. Because the vessel's orientation to the Earth's field changes, both the amount and direction of deviation can vary around the compass card.

A compass swing compares reliable known directions with compass readings on a range of headings, adjustments are made where appropriate, and the residual error is recorded on a deviation card. Use the entry for the intended compass heading and interpolate only as the card and procedure allow.

Treat the card as configuration-specific evidence, not a permanent property. Moving a speaker, adding electrical equipment, lightning, repairs or a material change from expected bearings warrants investigation and, where needed, a fresh check by a competent person. Keep portable magnetic items away during normal steering and observations.