Learn what magnetic variation is, how to convert true and magnetic courses, and why outdated or double-applied variation causes navigation errors.
Magnetic variation, also called magnetic declination, is the angle between true north and magnetic north at a particular place. It matters because charts, instruments, radio aids and avionics may reference different norths; failing to apply the correct east-or-west variation can turn an accurate true course into the wrong magnetic course.
What do east and west magnetic variation mean?
East or west variation describes where magnetic north lies relative to true north at your position. With easterly variation, magnetic north is east of true north; with westerly variation, it is west.
Variation is not constant worldwide. It changes with location and gradually with time because the Earth's magnetic field moves. Aeronautical charts may show isogonic lines or a compass rose with the variation, its reference year and an annual rate of change.
In aviation and real-world flying, magnetic variation provides the conversion between geographic directions and the magnetic directions commonly presented to pilots. Our separate explanation of true and magnetic references in flight simulators covers how those north references appear in the cockpit.
Why does magnetic variation matter in aviation?
Magnetic variation matters because mixing a true direction with a magnetic instrument produces a systematic navigation error. A 10° mistake maintained for 100 NM creates roughly 17 NM of cross-track displacement, even before wind is considered.
Variation can affect a heading, course or bearing, although those terms describe different things. If that distinction is unclear, see our practical explanation of how heading, course and bearing relate during a flight.
- Charts and procedures: Most conventional aviation courses and bearings are magnetic unless they are explicitly marked true. Entering a published magnetic course into an avionics system set to true creates an immediate mismatch.
- VOR navigation: VOR radials are normally tied to the station's calibrated magnetic declination. That station value can lag behind present-day local variation, so a published radial may not agree exactly with a GPS-derived magnetic bearing. Do not recalculate the published radial using the local variation; use the stated radial. Our guide to practical VOR and NDB tracking explains how to fly these indications.
- Runway numbers: Runway designations represent magnetic alignment rounded to the nearest ten degrees. Long-term movement of magnetic north can eventually require an airport to renumber a runway even though the pavement has not moved.
- Wind information: Weather reports and spoken cockpit or ATC information do not always use the same north reference. Check how ATIS wind direction is referenced before comparing it with a true-direction weather product.
How do you convert true and magnetic directions?
Convert true to magnetic by subtracting easterly variation or adding westerly variation; reverse the operation when converting magnetic to true.
| Conversion | Rule | Example |
|---|---|---|
| True to magnetic | Subtract east; add west | 090°T with 10°E becomes 080°M |
| Magnetic to true | Add east; subtract west | 080°M with 10°E becomes 090°T |
If easterly variation is treated as a positive number, the same rule can be written as M = T - V and T = M + V. Normalise the result to 000–359°; for example, 355° plus 10° becomes 005°, not 365°.
- Identify the reference. Look for labels such as °T, TRUE, °M or MAG. Never assume that two numbers use the same reference merely because they are shown on the same display.
- Find the applicable variation. Use the value for the relevant position and date. On a long route or near a strong variation gradient, the departure-airport value may not represent the entire leg.
- Convert only when necessary. If both the source course and the receiving instrument are magnetic, enter the course directly. The mistake we see constantly is applying variation twice because the GPS or FMS has already converted its internally calculated true track for display.
- Cross-check the result. A conversion should produce a plausible course relative to the charted route. A discrepancy of several degrees deserves investigation before departure rather than an extra correction chosen by guesswork.
Variation is not compass deviation
Variation comes from the Earth's magnetic field and changes with position and time. Compass deviation is an error caused by magnetic influences within the aircraft, such as electrical equipment and metal structure.
The correction sequence is true direction to magnetic direction using variation, then magnetic direction to compass direction using the aircraft's deviation data. A compass correction card addresses deviation; it does not replace the charted magnetic variation.
Why can a simulator disagree with a current chart?
A flight simulator can disagree with a current chart when the simulator, scenery, navigation database and aircraft avionics use magnetic data from different dates or sources.
- An older simulator may use a magnetic-field model fixed to an earlier epoch.
- Legacy airport scenery may retain an obsolete runway number after the real runway was renumbered.
- An add-on aircraft may calculate magnetic directions using its own model rather than the simulator's model.
- A VOR may correctly use its stored station declination while the GPS displays a direction based on present local variation.
- One instrument may be configured for true display while another remains magnetic.
First confirm the north reference on every display. For a published instrument procedure, fly the printed course unless it is explicitly marked true; do not “update” it with a variation taken from another chart. If only the runway number disagrees while the centreline heading is sensible, the likely problem is ageing scenery data rather than a faulty compass.