Compass Deviation Card

A vessel-specific table or graph showing magnetic-compass deviation for a range of headings.

Deviation is the angular error caused by the vessel’s own magnetic fields and is determined by swinging the compass and comparing known headings or bearings. The card records east or west corrections, often with separate values for principal and intercardinal headings and sometimes a curve for interpolation.

Electrical equipment, cargo, structural changes, lightning, repairs and time can alter deviation. The card must identify the compass and date, be checked periodically and distinguished from magnetic variation, which is geographic rather than vessel-specific.