Compass deviation
The error of the magnetic compass caused by the ship's own magnetic field — corrected via periodic swinging + adjustment.
Regulatory detail & full definition
Compass deviation is the compass error caused by the ship's own iron + steel + electrical fields distorting Earth's magnetic field at the compass position. Deviation varies with ship's heading + is tabulated in the Deviation Card. The Compass Adjuster performs periodic 'swinging' — sailing on set headings + comparing compass reading against a shore transit — to minimise + document the deviation.
Authoritative source: NGA Bowditch ↗
Related
Source: Bowditch American Practical Navigator
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