Magnetic variation and magnetic deviation are two separate sources of error that must be accounted for when using a magnetic compass, and confusing them is a common early mistake in navigation study.
Magnetic Variation
Variation is the angular difference between true north (the direction to the geographic North Pole) and magnetic north (the direction a compass needle points, toward Earth’s magnetic pole). Variation changes depending on your location on Earth and is shown on nautical charts for that specific area, since Earth’s magnetic field isn’t uniformly aligned with its geographic axis.
Magnetic Deviation
Deviation is a separate error caused by the ship’s own magnetic field — its steel structure, electrical equipment, and machinery — which distorts the compass reading aboard that specific vessel. Deviation varies by the ship’s heading (different headings expose the compass to different aspects of the ship’s magnetic field) and is documented in a deviation table or curve specific to each vessel and compass location.
Putting It Together
To convert between the three reference directions:
True direction ± Variation = Magnetic direction ± Deviation = Compass direction
Both errors must be applied, in the correct direction (east or west), to accurately convert between what the compass shows and the true direction needed for chart work.
Why This Still Matters With GPS and Gyro Compass
Even though gyro compasses (referenced to true north) and GPS have reduced day-to-day reliance on magnetic compass accuracy, every vessel still carries a magnetic compass as a mandatory backup — understanding and correctly applying variation and deviation remains a core, examinable navigation skill precisely because it’s the fallback when other systems fail.
Exam Relevance
Variation and deviation calculations, and correctly converting between compass, magnetic, and true directions, are foundational Navigation topics tested extensively in deck officer competency exams.