The algebraic sum of the variation and deviation is sometimes referred to as ______.

• A. Compass Error
• B. True Bearing
• C. Radar Fix
• D. Dead Reckoning Position

Answer: (A)

The idea being tested is how compass errors are formed from the two independent sources of error that affect a magnetic compass: variation and deviation. Variation is the angle between true north and magnetic north at a location, while deviation is the error caused by the vessel’s own magnetic fields. The algebraic sum of these two errors—taking east as positive and west as negative—gives the compass error. This compass error is the difference between what the compass reads (its bearing) and the magnetic bearing, so the relation is simple: compass bearing = magnetic bearing + compass error.

In practice, knowing both variation and deviation and adding them with the proper signs lets you translate between true, magnetic, and compass bearings. If the sum results in a positive value (east), the compass reads that many degrees high relative to magnetic bearing; if negative (west), it’s that many degrees low. This concept helps you correct readings to obtain accurate navigation bearings.