Which statement best describes magnetic variation and deviation?

• A. Variation is the angular difference between true north and magnetic north; deviation is the compass error caused by the vessel's magnetic field.
• B. Variation is corrected using a deviation table.
• C. Deviation is the angular difference between true north and magnetic north.
• D. Variation and deviation are the same concept.

Answer: (A)

Magnetic variation and deviation refer to two different sources of compass error. Variation is the angle between true north and magnetic north at a location, shown on nautical charts as the declination. Deviation is the compass error produced by the vessel’s own magnetic field, including the hull, machinery, and electrical systems, and it can change with the vessel’s course and equipment.

That combination is captured in the statement: variation is the angular difference between true north and magnetic north; deviation is the compass error caused by the vessel’s magnetic field. In practice, you use a deviation table to correct for deviation and a charted declination to correct for variation to arrive at a true heading. The other choices mix up these distinctions or treat them as the same concept.