Navigators rotate the ship to point at known magnetic headings to measure and correct compass deviation caused by the vessel's own magnetic field. Technicians adjust compensating magnets in the binnacle to cancel out interference from the steel hull and onboard electronics. Once adjusted, any remaining error is recorded on a deviation card for the crew to use during passage.
Heard on the job
We need to schedule a compass swing in the bay before we head offshore with the new radar installed.
Insider tip
A swing is required after major shipyard work or when carrying heavy magnetic cargo like scrap iron.