Which conversion uses Magnetic and Deviation to produce Compass?

• A. True ± Variation = Magnetic
• B. Variation + Deviation = Compass error
• C. Lining up objects to check position
• D. Magnetic ± Deviation = Compass

Answer: (D)

Understanding how to convert a magnetic heading to a compass heading by applying the vessel’s deviation is what this question focuses on. The compass reading you use on board is the magnetic heading adjusted for the ship’s own magnetic influences—the deviation. So the correct relationship is that the compass heading equals the magnetic heading plus the deviation (with the sign of deviation taken into account: add if the deviation is east, subtract if it’s west).

For example, if your magnetic heading is 100 degrees and the deviation is +5 degrees (east), the compass heading would be 105 degrees. If the deviation is -4 degrees (west), the compass heading would be 96 degrees. This shows how deviation directly modifies the magnetic reading to give the compass display.

Variation, on the other hand, is the difference between true north and magnetic north and is used to go between true and magnetic directions, not directly to the compass. The other options mix these concepts or describe actions that aren’t about converting magnetic to compass readings.