What is magnetic variation?

• A. The angular difference between true north and magnetic north; used to convert between true and magnetic bearings.
• B. The compass error caused by a vessel's magnetic field; corrected using a deviation table.
• C. The speed of a vessel through water measured at the hull.
• D. The geographic direction of true south.

Answer: (A)

Magnetic variation is the angular difference between true north and magnetic north. It tells you how far off a compass reading is from the geographic, or true, direction. On charts, true bearings come from the map’s grid, while a magnetic bearing comes from the compass. To convert between them, you apply the variation: if the variation is east, magnetic north lies east of true north, so you subtract the variation from the true bearing; if the variation is west, magnetic north lies west of true north, so you add the variation.

For example, with a true bearing of 090° and a variation of 10° East, the corresponding magnetic bearing would be 080°. If the variation is 10° West, the magnetic bearing would be 100°. Variation values are published on nautical charts and change over time as the Earth’s magnetic field shifts, so using the latest data is important.

The other option describes deviation, which is caused by a vessel’s own magnetic field and is corrected with a deviation table, not magnetic variation. The remaining options refer to speed through water or the direction of true south, which aren’t what magnetic variation measures.