Magnetic variation, deviation & compass error

Magnetic variation (also called declination) is the angle between true north and magnetic north at a given position. It varies geographically — from near zero on the agonic line to over 30° in the high arctic — and changes year by year as the Earth's magnetic field drifts. Variation is the largest correction the bridge applies to every gyro-failed course.

Definitions

Variation — true bearing − magnetic bearing. East variation is positive (magnetic north is east of true north).
Deviation — magnetic bearing − compass bearing. Caused by the ship's own magnetism; varies with heading; recorded in the deviation card.
Compass error — variation + deviation (true − compass). Always apply with sign awareness.
Annual change — variation increases or decreases by a small amount each year as the magnetic field drifts; the chart legend states the annual change.

CADET → working rule

CDMV(T) — Compass + Deviation = Magnetic + Variation = True

East = positive (add); West = negative (subtract). Going compass → true: add easterly errors, subtract westerly. Going true → compass: reverse.

Mnemonic: Cadets Drink More Vodka — True! for the order; Add East going up for the sign.

Reading the chart compass rose

Every paper chart and ENC carries one or more compass roses showing variation at the rose centre, the year of survey, and the annual change. To get current variation:

Var (now) = Var (chart year) + (year_now − year_chart) × annual_change

Example: chart shows 4°15'W (2018), annual change 5'E. In 2026: 4°15' − 8 × 5' = 3°35'W.

Charts more than ten years out of date risk being too far off for safe navigation in high-variation zones — request a new edition or use ECDIS with current ENC updates.

The World Magnetic Model

The WMM is a joint NOAA / BGS / NGA product released every five years. WMM2025 (valid 1 January 2025 – 31 December 2029) is the current version. It models the Earth's main magnetic field as a 13×13 spherical harmonic series with annual secular-variation terms.

For operational accuracy at any point and date, use the official online calculator at ncei.noaa.gov/products/world-magnetic-model or the BGS calculator at geomag.bgs.ac.uk/data_service/models_compass/wmm_calc.html. Both also publish the underlying coefficients for offline tools.

Areas of rapid change

The North Magnetic Pole has moved from northern Canada (1990) toward Siberia at ~55 km/year. Charts of the high-arctic Northwest Passage need frequent updating.
The South Atlantic Anomaly (off Brazil) is weakening rapidly; total field intensity here can fall ~5% per decade.
Equatorial Africa shows the fastest rate of variation change globally — annual change of 10'–15'.

Compass deviation card & swinging the compass

Deviation is determined by "swinging the compass" — heading the ship through cardinal and inter-cardinal directions and comparing magnetic compass to true. Performed:

After installation or major repair to magnetic compass.
After a long voyage in high latitudes.
After significant change in cargo (steel cargo affects deviation).
Annually for SOLAS V/19.2.1.1 vessels — recorded in the Compass Deviation Book.

Why no live calculator here

We deliberately don't embed a WMM calculator on this page. The full WMM2025 coefficient set is large (~340 numbers) and any subset gives errors of several degrees that could produce the wrong magnetic bearing for navigation. The authoritative NOAA and BGS calculators above are free, accurate, and updated as soon as the model is revised. Use them.