Deviation Card

A deviation card is a calibrated table or curve showing the residual deviation of a vessel's magnetic compass on each heading from 000° to 360°. It is prepared after a compass swing, during which the vessel is turned slowly through all points of the compass in sheltered water and the deviation on each heading is recorded by comparing the compass reading with a known true heading derived from a transit, gyrocompass bearing, or GPS course over the ground. Bowditch (American Practical Navigator) and the Admiralty Manual of Navigation describe the swinging procedure and the graphical interpolation methods used to derive the tabulated or curved form. The OOW uses the deviation card by entering with the ship's compass heading, reading off the deviation applicable to that heading, and applying it algebraically to convert compass to magnetic bearing. The card should be posted at the compass and renewed whenever a compass adjuster re-swings the ship. Cards become unreliable after structural alterations, cargo changes involving steel or magnetised material, proximity to large magnets, or simply the passage of time due to slow changes in the ship's magnetic signature. During Port State Control inspections, surveyors check that the deviation card is current and signed; an absent or outdated card is grounds for deficiency notation.