Compass error is the total angular difference between the direction a magnetic compass indicates as north and true north — the combined result of two genuinely distinct sources of error that must be calculated separately, then combined.
The Two Components
- Variation — the difference between magnetic north and true north caused by the Earth’s own magnetic field, which varies by geographic location and is shown on nautical charts for that specific area
- Deviation — the additional error caused by the ship’s own magnetism (steel hull, electrical equipment) affecting the compass needle, which varies depending on the ship’s heading at the time and is recorded on a deviation card specific to that vessel
The Formula
Compass Error = Variation ± Deviation
Both variation and deviation can be either easterly or westerly, and the sign convention (adding or subtracting) depends on the direction of each component — a genuinely common source of navigation calculation errors if applied carelessly.
Why This Distinction Matters
Variation changes based on where the ship is in the world (and is charted accordingly), while deviation changes based on the ship’s own heading (and is unique to that specific vessel) — conflating the two, or applying only one when both apply, produces a genuinely incorrect true bearing.
Practical Use
Converting between compass, magnetic, and true bearings using variation and deviation is a routine, practical navigation task — not just an exam topic — essential whenever plotting a compass-derived bearing onto a chart that’s referenced to true north.
Exam Relevance
Compass error calculation, correctly distinguishing variation from deviation, and converting between compass/magnetic/true bearings are core, heavily-tested Navigation topics across all levels of deck officer competency exams.