Compass Errors: Variation, Deviation and Gyro Error

Magnetic poles and variation

In short: Read bridge equipment and SOLAS V/19 first: it tells you which compasses a ship carries. This topic is about the errors of those compasses. Variation is the angle between true north and magnetic north. It belongs to the place, not to the ship.

• Geographic (true) poles: the ends of the Earth’s spin axis. They do not move. Charts and the gyro work to true north.
• Magnetic poles: where the Earth’s magnetic field points straight down. They are not at the geographic poles and they wander year by year.
• A magnetic compass needle lines up with the horizontal part of the Earth’s field. Near the magnetic poles that part is weak, so the compass becomes sluggish and unreliable.
• Variation = angle between true north and magnetic north, named East or West. It changes with position and slowly with time.
• Lines joining places of equal variation are isogonic lines; the line of zero variation is the agonic line.

Reading variation from the chart

• The compass rose shows the variation, the year it applies to and the annual change.
• Bring it up to date: years elapsed × annual change, then apply it.
• Example: rose reads Var 4°15’W (2015), annual change 8’E. In 2026: 11 × 8’ = 88’ = 1°28’ E. 4°15’W − 1°28’ = 2°47’W, use 3°W.
• On ECDIS, query the chart for the magnetic variation in the same way.

Deviation: causes and correctors

In short: Deviation is the angle between magnetic north and compass north, caused by the ship’s own magnetism. It comes in two kinds, and the compass adjuster cancels each one with its own corrector.

• Flinders bar position, as usually taught: bridge aft → bar fitted forward of the compass; bridge forward → bar fitted aft. The adjuster sets the exact side and length for the ship’s own iron.
• Heeling error: when the ship heels or rolls, the iron moves relative to the compass. A vertical heeling magnet under the bowl corrects it.
• Retention error: after a long time on one heading (or alongside), the ship keeps some of that magnetism for a while after a course change.
• Gaussin error (often written “Gaussian”): a short-lived extra deviation just after a large alteration of course, while the induced magnetism settles.
• Where the compass lives: usually on the monkey island above the bridge, on the centreline, as far as possible from magnetic interference. Its card is reflected down into the wheelhouse.

Why deviation changes with the ship’s head

In short: The Earth’s field stays put; the ship’s magnetism turns with the ship. On every heading the ship’s field pulls on the needle from a different angle, so deviation is different on every heading. That is why a deviation card is a table by ship’s head.

• Permanent (hard-iron) magnetism gives semicircular deviation: one sign over half the headings, the other sign over the other half.
• Induced magnetism in horizontal soft iron gives quadrantal deviation: it changes sign every 90°, largest on the intercardinal headings (NE, SE, SW, NW).
• Vertical soft iron gives a deviation that changes with magnetic latitude — the reason for the Flinders bar, and why deviations shift after a big change of latitude.
• On the bridge: always take deviation for the heading you are steering, never for the bearing you are taking.

Adjusting and swinging the compass

In short: Each magnetic compass must be properly adjusted, with its table or curve of residual deviations available at all times (MGN 610 (M+F) Annex D). Adjustments are made by a holder of a Certificate of Competency as Compass Adjuster (MGN 610 (M+F) Annex D; the certificate itself: MGN 279 (M)).

Swinging the ship

1. The ship is steadied on a series of headings all round the compass (at least the cardinal and intercardinal points).
2. On each heading the compass reading is compared with the correct magnetic heading (from transits, the Sun’s azimuth or a corrected gyro).
3. The adjuster places the magnets, spheres and Flinders bar to reduce the deviations.
4. The residual deviations go on the deviation card; the date and the position of the correctors go in the compass deviation book (MGN 610 (M+F) Annex D).

• A transmitting magnetic compass gets its own deviation card, separate from the standard compass. MGN 610 warns it can otherwise end up over-compensated, with an error of up to 2½°.

When to adjust or check

Adjust when (MGN 610 (M+F) Annex D 1.3.2):

1. The compass is first installed (new build).
2. It becomes unreliable (deviations do not match the card).
3. The ship has structural repairs or alterations that could change its magnetism (for example welding near the compass, or fire or damage).
4. Electrical or magnetic equipment near the compass is added, removed or altered (major items fitted or taken off).
5. Two years have passed since the last adjustment and no deviation record has been kept, or deviations are excessive, or the compass has physical defects.

• You will often hear simply “every 2 years”. MGN 610 ties the two years to the state of the deviation record: a ship that logs its deviations and finds them small is not forced to re-adjust.
• Check performance particularly after (MGN 610 (M+F) Annex D 1.4.2): carrying magnetic cargo; loading with electromagnetic lifting gear; a casualty with severe contact or electrical charges; being laid up or idle (even a short period, especially on small vessels — so a long spell alongside counts). Watch it closely in a new ship’s early life.
• Also check deviations after a major change of latitude: the induced part of the deviation changes with magnetic latitude.

Daily condition checks: gimbals free · card floating freely · no air bubble · numbers clearly readable · optical (reflector) system working · light working.

Compass error: converting compass and true

In short: Compass error = variation + deviation, combined by name: same names add, different names subtract. Then: compass → true, add East, subtract West.

• Mnemonic CADET: Compass to true, ADd EasT (and subtract West).
• Mnemonic for the name of the error: “Error East, compass least; error West, compass best.”
• Order of the steps: Compass → (deviation) → Magnetic → (variation) → True. Going the other way, reverse every sign.

Worked examples

• Course, compass to true: heading 095°C, deviation for that head 2°E, variation 3°W. Error = 2°E − 3°W = 1°W. True = 095° − 1° = 094°T. (Step by step: 095°C + 2°E = 097°M; 097°M − 3°W = 094°T.)
• Course, true to compass: you want 230°T; variation 3°W, deviation for that head 4°W. Error = 7°W. Compass = 230° + 7° = 237°C.
• Bearing: still heading 095°C, a lighthouse bears 310°C. Use the error for the ship’s head (1°W), not for 310°: true bearing = 309°T.

Checking compass error on the bridge

In short: Find the error before departure, at least once a watch, and after every large alteration of course (MGN 610 (M+F) Annex D 1.5.1). It is part of the safe navigational watch, and gyro and magnetic compass errors are passed on at the watch handover.

Methods

• Transit: two charted objects come in line. Read the transit’s true bearing off the chart, take a compass bearing as they line up: the difference is the compass error.
• Alongside, pre-departure: lying square to the quay, compare the ship’s heading with the charted direction of the quay.
• Charted known bearing: a leading line or a mark whose true bearing you know.
• Sun’s azimuth at any time (azimuth tables or calculation) or amplitude at sunrise and sunset (next section).
• Against the gyro: gyro heading corrected for gyro error, then apply variation → correct magnetic heading. Compare with the magnetic compass to get deviation for that head.
• Record every check in the compass deviation book (MGN 610 (M+F) Annex D 1.5).

Bearing instruments

• Pelorus: sighting ring on a magnetic compass. It gives relative bearings to the ship’s head.
• Azimuth mirror (or circle) on a gyro repeater:
  • arrow up for a heavenly body;
  • arrow down for a terrestrial object;
  • keep the prism horizontal (watch the bubble).

Amplitude: compass error by the Sun

In short: At sunrise and sunset the Sun’s true bearing depends only on its declination and your latitude. Take a bearing, look up the true one, and the difference is your compass or gyro error.

1. Take the bearing when the Sun’s lower limb is about half a diameter above the visible horizon (its centre is then on the true horizon).
2. Find the Sun’s declination for the date in the daily pages of the Nautical Almanac.
3. Note your latitude.
4. Get the amplitude from Norie’s Nautical Tables (or a rising/setting azimuth table). By calculation: sin amplitude = sin declination ÷ cos latitude.
5. Name it: E at sunrise, W at sunset, then N or S like the declination. Example: E 11° N = true bearing 090° − 11° = 079°T.
6. Compare the true bearing with the observed one: the difference is the error.

Worked examples

• Gyro error at sunrise: Sun bears 081°G. Latitude 24°N, declination 10°N. Amplitude = E 11.0° N → true 079°T. The gyro reads 2° more than true: gyro error 2° High.
• Compass error at sunset: Sun bears 259°C. Latitude 34°N, declination 5°S. Amplitude = W 6.0° S → true 270° − 6° = 264°T. True is 5° more than compass: compass error 5°E.

Gyro compass and gyro error

In short: A gyro compass finds true north with a spinning rotor, the Earth’s rotation and gravity. It has no variation and no deviation, but it has errors of its own, so you still check it every watch. Required from 500 GT with heading and bearing repeaters (SOLAS V/19.2.5).

• Speed (and course) error: the ship’s north–south speed adds to the Earth’s rotation, so the gyro settles slightly off the meridian. Worse at high speed, high latitude and on northerly or southerly courses. Corrected by feeding in speed and latitude.
• Latitude error: the settling point also shifts with latitude. That is why the gyro takes a latitude (GPS) input.
• Ballistic deflection: a temporary error during big changes of speed or course.
• The gyro needs power and time to settle after start-up: switch it on well before sailing. The magnetic compass is the back-up that needs neither.
• Naming gyro error: gyro reads more than true → error High → subtract it. Gyro reads less than true → error Low → add it.
• A wrong heading input spoils everything fed by the gyro: radar and ARPA vectors in radar plotting, every bearing in a radar fix. Before fixing or monitoring by radar, check the gyro error and the heading marker alignment (MGN 379 (M+F)), as in parallel indexing.
• Gyro error is found by the same methods as compass error: transits, charted bearings, Sun’s amplitude or azimuth.

Transmitting heading devices and GPS compasses

In short: From 300 GT a ship carries a transmitting heading device (THD) that sends heading to the radar, plotting aid and AIS (SOLAS V/19). It can be a transmitting magnetic compass, a gyro or a satellite (GPS) compass.

• A transmitting magnetic compass has its own deviation card (see swinging, above). Performance standard for transmitting magnetic heading devices: IMO MSC.86(70), annex 2; magnetic compasses: IMO A.382(X) (both cited in MGN 610 (M+F) Annex D).
• GPS (satellite) compass: receives satellite signals on two or more antennas (often three) a known distance apart and works out heading from their positions relative to each other.

• On ships over 500 GT the heading must also be readable at the emergency steering position (MGN 610 (M+F) Annex D).