What is magnetic deviation?

• 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 distance between the center of gravity and the metacenter.
• D. The rate of change of the Earth's magnetic field.

Answer: (B)

Magnetic deviation is the compass error caused by the ship’s own magnetic field. The iron, steel, and electrical gear on a vessel create magnetic influence that shifts the compass reading away from magnetic north, and this deviation changes with the ship’s heading. To correct for it, mariners use a deviation table that lists the necessary heading-specific corrections to apply to the compass reading.

In contrast, the angular difference between true north and magnetic north is called variation, not deviation, and is used for converting true bearings to magnetic bearings. The distance between the center of gravity and the metacenter relates to stability (GM), not navigation bearings. The rate of change of Earth’s magnetic field is secular variation, a longer-term change rather than a ship-specific compass error.