Deviation Cards and Swinging the Compass

A deviation card is a table that records your boat's compass deviation on a range of headings — typically every 15° or 30° around the full 360°. It is specific to your vessel, and must be recalculated whenever you make significant changes to equipment near the compass (installing new electronics, moving the engine battery, adding speakers, etc.).

Swinging the compass is the procedure used to create a deviation card. The boat is motored slowly in a circle, pausing on each cardinal and intercardinal heading (and ideally every 15°–30°). On each heading, the helmsman compares the compass reading with a known reference bearing — usually a transit (two fixed charted objects in line). The difference between the compass bearing of the transit and the charted magnetic bearing of the transit gives the deviation for that heading.

Alternatively, a pelorus (a dumb compass card without a magnet, aligned to the ship's head) or a GPS-derived heading can serve as the reference. A qualified compass adjuster can also swing the compass and place corrector magnets inside the binnacle to minimise deviation before creating the final card.

To use the deviation card at sea, look up the compass heading you are steering and read off the corresponding deviation. If your heading falls between two tabulated values, interpolate between them. For example, if deviation is 2°E on 030° and 3°E on 060°, a heading of 045° would have approximately 2.5°E deviation.