The compass error caused by the boat's own magnetic fields, different on every heading.
Deviation is the error of a boat's compass caused by magnetic influences on board: the engine, steel parts, speakers, electronics, even a phone or a tin of food placed near the compass. It deflects the needle away from magnetic north. Unlike magnetic variation, deviation is specific to each boat and each compass, and it changes with the boat's heading.
The deviation table
Because deviation depends on the heading, it's recorded in a deviation table (or deviation card): the error for every 15° or 30° of heading, east or west. It's determined by swinging the compass: sailing on known headings and comparing the compass reading with a reference such as transit marks or GPS. A compass adjuster can reduce large deviation with correcting magnets.
From compass to true
| Step | Correction |
|---|---|
| Compass course | + deviation |
| = Magnetic course | + variation |
| = True course |
East is added, west subtracted. A classic memory aid for the order: Can Dead Men Vote Twice (compass, deviation, magnetic, variation, true).
Electronic fluxgate compasses usually calibrate themselves when you turn slow circles with the boat. Keep loose metal and electronics away from any compass, and check its deviation again after installing new equipment near it.