Why compasses need correcting
A magnetic compass points to magnetic north, not true north, and is further deflected by the vessel's own magnetic field — permanent magnetism baked into the hull and machinery, and induced magnetism that shifts with the ship's heading and latitude. The Master's responsibility is to ensure the compass is corrected as closely as practicable and that any residual error is known, recorded, and applied.
The two errors
Variation is the angular difference between true north and magnetic north at a given position. It is caused by the Earth's field alone, is shown on the chart, and changes with geographic position and over time (annual change is stated on the compass rose). The navigator corrects for it using the chart — it cannot be adjusted out of the compass.
Deviation is caused by the vessel's own magnetic field acting on the compass needle. It varies with the ship's heading and, to a lesser extent, with magnetic latitude and changes in the vessel's magnetic character (cargo, structural alterations, time). Deviation is addressed in two ways: physical correction using correctors, and residual deviation recorded on a deviation card.
Compass correctors
The standard compass binnacle carries several corrector systems:
- Flinders bar — soft iron bar mounted vertically (fore or aft), corrects for vertical induced magnetism that changes with latitude and heel.
- Quadrantal correctors (spheres) — soft iron spheres mounted athwartships, correct for quadrantal deviation (induced horizontal magnetism), which peaks on intercardinal headings.
- Fore-and-aft and athwartships magnets — permanent magnets in the binnacle, correct for the constant component of deviation caused by the vessel's permanent magnetism. These address semicircular deviation, which changes sign as the ship swings.
- Heeling magnet — mounted vertically below the compass, corrects for heel-induced deviation.
Adjustment (swinging the compass) is carried out by a qualified compass adjuster and should be repeated after any significant structural work, changes to the vessel's magnetic character, or relocation of large ferrous equipment. As Master you commission the swing — you are responsible for ensuring it has been done and the results are valid.
The deviation card and its use
After adjustment, residual deviation on each cardinal and intercardinal heading is recorded on a deviation card, retained on the bridge and applied on every heading. The Master must ensure the card is current and that watchkeepers actually use it. If deviation on any heading exceeds an acceptable margin (typically referenced in the vessel's SMS), the compass must be re-swung.
Applying corrections: the master's decision
The Master must be able to convert between true, magnetic, and compass bearings using the mnemonic CADET (Compass + deviation = Magnetic; Magnetic + variation = True) or its inverse, and must recognise when accumulated error renders the compass unreliable. Monitoring compass error regularly — by transit bearing, celestial azimuth, or GPS COG on a steady heading — is a command-level habit, not a watchkeeper's tick-box.