Compass error is the combination of what?

• A. Deviation and Variation
• B. Deviation and Heading
• C. Variation and Heading
• D. Heading and Depth

Answer: (A)

Compass error comes from two separate magnetic effects acting together: variation and deviation. Variation is the difference between true north and magnetic north at your location—the angle you read on charts that tells you how the Earth’s field is oriented there. Deviation is the additional deflection caused by the ship’s own magnetic influence—steel hull, engines, equipment, and nearby magnets—which changes with your heading. The total error you see on the compass is the combination of these two effects, usually treated as the sum (with the appropriate sign) of variation and deviation. This is why charts give the variation and the ship’s deviation table provides the deviation for each heading. Heading and depth aren’t sources of compass error themselves, though the heading you choose will be corrected by the compass error.