The error in a magnetic compass caused by the ship's own magnetic field, varying with the vessel's heading.
In practice
Deviation changes over time as the ship's magnetic signature evolves through voyages, cargo changes, welding, and proximity to strong magnets. A compass adjuster should swing the ship and update the deviation card after any significant structural work, following drydock, or when the residual deviation on any heading exceeds a few degrees. Electrical equipment, loudspeakers, and mobile phones near the compass also induce temporary deviation. Ignoring deviation—or using an outdated deviation card—introduces a systematic error in all compass-derived bearings and courses, which can accumulate into a dangerous position error on a long leg without external position checks.
Regulatory detail & full definition
Deviation is the angular error in a magnetic compass caused by the vessel's own permanent and induced magnetic field, which deflects the compass needle away from magnetic north. Unlike variation, which is a fixed property of the earth at a given location, deviation changes with the ship's heading and must be measured on every compass heading during a compass swing. The results are tabulated on a deviation card, which the OOW uses to apply the deviation correction appropriate to the current heading when converting compass bearings to magnetic. Bowditch (American Practical Navigator) covers compass adjustment and residual deviation in detail.
Deviation changes over time as the ship's magnetic signature evolves through voyages, cargo changes, welding, and proximity to strong magnets. A compass adjuster should swing the ship and update the deviation card after any significant structural work, following drydock, or when the residual deviation on any heading exceeds a few degrees. Electrical equipment, loudspeakers, and mobile phones near the compass also induce temporary deviation. Ignoring deviation—or using an outdated deviation card—introduces a systematic error in all compass-derived bearings and courses, which can accumulate into a dangerous position error on a long leg without external position checks.