A line drawn on a chart gives you a true course, measured from true north. Your compass, your heading indicator and most runway numbers work in magnetic degrees. Turning one into the other takes three corrections: wind, variation and deviation. Pilots learn the order with the phrase “True Virgins Make Dull Company”, and the direction of each correction with “East is least, West is best”. Work out wind correction with the Wind Correction Angle Calculator.
The chain from chart to compass
| Step | Correction | Result |
|---|---|---|
| True course (TC) | ± wind correction angle (WCA) | True heading (TH) |
| True heading | ± magnetic variation (VAR) | Magnetic heading (MH) |
| Magnetic heading | ± compass deviation (DEV) | Compass heading (CH) |
Some courses apply variation first to get a magnetic course, then the wind correction. The result is the same as long as every correction is applied once with the correct sign.
Variation: true north vs magnetic north
Magnetic north is not at the geographic North Pole, and the Earth’s magnetic field is uneven. The angle between true north and magnetic north at a given place is the magnetic variation (called declination outside aviation). Charts show it with isogonic lines. It can be a few degrees or more than 20°, and it changes slowly over the years, which is one reason airports occasionally renumber runways.
- Westerly variation: add it. True 090°, variation 5°W → magnetic 095°. “West is best.”
- Easterly variation: subtract it. True 090°, variation 5°E → magnetic 085°. “East is least.”
The rule works going from true to magnetic. Going the other way, reverse the signs.
Deviation: your aircraft’s own error
Metal, wiring and electronics near the compass bend its reading slightly. That error, which changes with heading, is the deviation. It is measured during a compass swing and written on the compass correction card next to the instrument. A typical card reads something like “For 090, steer 088”.
Worked example
- True course from the chart: 090°
- Wind correction angle: 8° right, because the wind is from the right
- Variation: 5°W
- Compass card: for 100°, steer 102°
- True heading: 090 + 8 = 098°
- Magnetic heading: 098 + 5 (west is best) = 103°
- Compass heading: the card for headings near 100° shows +2, so 103 + 2 = 105°
Flying 105° on the compass keeps the aircraft on the 090° true course drawn on the chart.
Where true and magnetic are used
- Magnetic: runway numbers, VOR radials, airways in most of the world, ATC headings, surface winds given by ATC and ATIS.
- True: chart-plotted courses, winds in TAFs and METARs, winds aloft forecasts, and in many countries the whole system in high-latitude areas where the compass is unreliable.
The mismatch between true winds in METARs and magnetic winds from the tower is a classic trap when working out a crosswind with the Crosswind Calculator. Where variation is large, convert first.
Common mistakes
- Applying “west is best” in the wrong direction. It applies going from true to magnetic.
- Applying variation twice, once to the course and again to the heading.
- Using old variation from an out-of-date chart. Variation drifts over time. Use current charts.
- Ignoring deviation on a long leg. Two degrees over 100 NM is a lateral error of about 3.5 NM.
FAQ
Is magnetic variation the same as declination?
Yes. Aviation and marine navigation say variation; surveying and geology usually say declination.
Why are METAR winds true but ATIS winds magnetic?
Written reports and forecasts are made for many users, so they use true north. Winds spoken by ATC and ATIS are for pilots lining up with magnetic runways, so they are magnetic. Local practice varies, so check your State’s AIP.
Does GPS make variation irrelevant?
No. GPS can show true or magnetic values, but runways, radials and ATC instructions are still magnetic, and the compass is still your back-up.
Sources and further reading
FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25), chapter on navigation; ICAO Annex 3 on how wind is reported. Use current aeronautical charts for variation values.
Last reviewed: November 2026.