What is compass deviation and how does it differ from variation?

Variation is the angle between True North and Magnetic North at a given location, caused by the Earth's magnetic field. It is shown on every nautical chart and changes slowly with time. Deviation is the additional error in the ship's compass caused by the vessel's own magnetic field — from the hull, engines, and cargo. Deviation changes as the ship swings to different compass headings and must be determined by swinging the ship.

Why does deviation change with heading?

The ship's permanent and induced magnetism creates a local field that adds to or subtracts from the Earth's field depending on the compass needle's orientation relative to the ship's magnetic axis. When the ship heads north, the ship's fore-aft field aligns with the Earth's field; when heading east or west, the athwartships component is at its maximum. This produces the sinusoidal deviation curve characteristic of deviation tables.

When should a vessel swing for deviation?

IMO recommends swinging at least every two years and after any event likely to change the ship's magnetic character: dry-docking, heavy structural welding, significant cargo changes (especially magnetic ores or steel), lightning strikes, or following a change in the degaussing coil settings. SOLAS V/19 requires a proper deviation table or curve to be maintained.

What are Fourier Coefficients B and C?

Deviation is analysed into Fourier harmonics: Coefficient B is the amplitude of the component that is maximum when heading East or West (090/270), caused mainly by the ship's permanent fore-aft magnetism. Coefficient C is the amplitude of the component maximum at North or South (000/180), caused mainly by athwartships permanent magnetism. A large B or C indicates a strong permanent magnet that the compass adjuster can partially compensate using corrector magnets.

Deviation table calculator

To swing for deviation, steer each compass heading in turn and observe the compass bearing of a distant object (or transit) whose true bearing is known. Enter the true bearing, local variation (E positive, W negative), and the compass bearing observed on each heading below. The calculator derives the magnetic bearing of the object and the deviation on each heading.

True bearing of distant object (°T)

From chart, transit, or GPS bearing — true north reference.

Variation (° — E positive, W negative)

From the nearest compass rose on the chart.

Compass bearings of the object on each heading

Steer each heading steady, then read and enter the compass bearing to the distant object.

Hdg 000° — bearing (°C)

Hdg 045° — bearing (°C)

Hdg 090° — bearing (°C)

Hdg 135° — bearing (°C)

Hdg 180° — bearing (°C)

Hdg 225° — bearing (°C)

Hdg 270° — bearing (°C)

Hdg 315° — bearing (°C)

Deviation table

Magnetic bearing of object: 40.0°M

Compass Hdg	Compass Bg	Magnetic Bg	Deviation
000°	—	40.0°M	—
045°	—	40.0°M	—
090°	—	40.0°M	—
135°	—	40.0°M	—
180°	—	40.0°M	—
225°	—	40.0°M	—
270°	—	40.0°M	—
315°	—	40.0°M	—

Coefficient B (E/W max): —
Coefficient C (N/S max): —

Notes & caveats

· The object used for swinging must be at least 3–5 nm distant so that bearing changes as the ship rotates around the anchor are negligible. Closer objects give significant parallax errors.
· Coefficients B and C are first-order Fourier estimates only. A complete analysis (Coefficients A through E) requires the deviation on all eight headings and is performed by a compass adjuster using the full Fourier decomposition.
· Deviation values here apply to the standard compass only. Flux-gate and gyro compasses have different error modes; consult the manufacturer's calibration procedure.
· The mnemonic for applying errors is TVMDC (True, Variation, Magnetic, Deviation, Compass) — 'add West' going left to right (True to Compass); 'add East' going right to left.

Sources: Admiralty Manual of Navigation Vol 2; SOLAS V/19; see also passage time & ETA, anchor scope calculator, compass deviation.

Deviation table calculator

True bearing of distant object (°T)

From chart, transit, or GPS bearing — true north reference.

Variation (° — E positive, W negative)

From the nearest compass rose on the chart.

Compass bearings of the object on each heading

Steer each heading steady, then read and enter the compass bearing to the distant object.

Hdg 000° — bearing (°C)

Hdg 045° — bearing (°C)

Hdg 090° — bearing (°C)

Hdg 135° — bearing (°C)

Hdg 180° — bearing (°C)

Hdg 225° — bearing (°C)

Hdg 270° — bearing (°C)

Hdg 315° — bearing (°C)

Deviation table

Magnetic bearing of object: 40.0°M

Compass Hdg	Compass Bg	Magnetic Bg	Deviation
000°	—	40.0°M	—
045°	—	40.0°M	—
090°	—	40.0°M	—
135°	—	40.0°M	—
180°	—	40.0°M	—
225°	—	40.0°M	—
270°	—	40.0°M	—
315°	—	40.0°M	—

Coefficient B (E/W max): —
Coefficient C (N/S max): —

Notes & caveats

· The object used for swinging must be at least 3–5 nm distant so that bearing changes as the ship rotates around the anchor are negligible. Closer objects give significant parallax errors.

· Coefficients B and C are first-order Fourier estimates only. A complete analysis (Coefficients A through E) requires the deviation on all eight headings and is performed by a compass adjuster using the full Fourier decomposition.

· Deviation values here apply to the standard compass only. Flux-gate and gyro compasses have different error modes; consult the manufacturer's calibration procedure.

· The mnemonic for applying errors is TVMDC (True, Variation, Magnetic, Deviation, Compass) — 'add West' going left to right (True to Compass); 'add East' going right to left.