compass swing
The process of swinging and compensating a ship or aircraft compass by determining and reducing the deviation coefficients and recording the residual deviations; now done by computer.
compass swing: correcting magnetic errors on a vessel
A compass swing is the systematic process of testing and correcting a ship's magnetic compass to account for the magnetic field distortions caused by the vessel's own metal structure and equipment. Every steel or iron ship generates local magnetic anomalies that deflect the compass needle away from true magnetic north. A swing determines how much the compass deviates at different headings and applies physical or electrical corrections to minimize these errors.
The process involves taking the vessel through a series of known headings, typically 8, 16, or 32 points around a full circle, while comparing the compass reading to a known reference direction. This reference may be an external magnetic bearing to a distant object, GPS/GNSS positioning, or a theodolite measurement. Each heading reveals the deviation: the difference between what the compass shows and the actual direction the ship is pointing. These measurements are plotted to create a deviation curve that shows which headings produce the largest errors.
Correction happens in two main ways. Permanent magnets mounted near the compass can be adjusted to cancel out fixed magnetic fields created by the ship's structure. Soft iron correctors, typically spheres or rods of unmagnetized iron, reduce the effects of induced magnetism, which varies with heading. Modern vessels use electronic compensation built into the compass system itself, applying mathematical corrections calculated by onboard software rather than physical adjustment.
The residual deviation, the error that remains after compensation, must be recorded in a deviation table or card and posted at the compass. This allows watchkeepers to apply manual corrections when steering by compass. Well-executed swings typically reduce deviations to less than 2 or 3 degrees across most headings. Poor swings or uncorrected compasses can introduce errors of 10 degrees or more, creating serious navigation risks.
Swings must be performed when a compass is first installed, after major hull repairs or alterations that change the magnetic environment, or if unexplained compass errors appear during operation. The term originated from the deliberate swinging motion: the ship was steered through a complete circle to sweep through all headings, with deviations recorded throughout. Modern computerized systems still follow this principle but automate the measurement and calculation work that once required hand plotting and manual arithmetic.