Magnetic compass errors are a fundamental topic on every USCG deck license exam. There are two components: variation (the difference between True North and Magnetic North, caused by the Earth's magnetic field) and deviation (the difference between Magnetic North and Compass North, caused by the vessel's own magnetic field). The mnemonic TVMDC — True, Variation, Magnetic, Deviation, Compass — is the standard memory aid.
Applying the corrections
The rule is: Can Dead Men Vote Twice? (Compass, Deviation, Magnetic, Variation, True). When converting from Compass to True, add Easterly errors and subtract Westerly errors. When converting from True to Compass, reverse: add Westerly errors, subtract Easterly. Confusing these directions accounts for a large share of wrong answers on navigation exams.
- Variation is fixed for a given location and changes very slowly (by a fraction of a degree per year)
- Deviation changes with the ship's heading — that's why you swing the compass
- A deviation table lists the compass error for each magnetic heading, taken at 15° or 30° intervals
Compass swinging procedure
To swing a magnetic compass, the vessel is turned slowly through 360° (usually by conning it in a circle in calm water with no current). At each 15° or 30° interval, the compass heading is recorded and compared to a known reference (usually a range, GPS, or sun azimuth). The difference is the deviation for that heading.