In navigation, what does Variation describe?

• A. Difference between true north and grid north
• B. Angle between magnetic north and compass needle
• C. Time variation of Earth's magnetic field
• D. Difference between true and magnetic north

Answer: (D)

Variation is the angular difference between true north (geographic north) and magnetic north (the direction the compass points). It tells you how far magnetic north is offset from true north at your location. The value is labeled as East or West depending on which way magnetic north lies relative to true north: if magnetic north is east of true north, Variation is East; if it’s west, Variation is West.

You use this to convert bearings between true and magnetic references. For example, with Variation East, a magnetic bearing of 90° corresponds to a true bearing of 102° (90° + 12°). With Variation West, a magnetic bearing of 90° would correspond to a true bearing of 80° (90° − 10°). Variation is charted for each area and changes slowly over time as Earth's magnetic field changes.

This differs from the angle between the magnetic north and the compass needle (the needle aligns with magnetic north itself), from grid north differences on maps (grid variation), and from the time variation of Earth’s magnetic field (secular variation).