Compass error

Compass error is the total angle between true north and compass north, the algebraic sum of magnetic variation and compass deviation. Each component is signed: easterly errors are added when converting compass to true, westerly errors subtracted. So compass error equals variation plus deviation, and true course equals compass course plus easterly compass error. The navigator checks total compass error directly by comparing the compass bearing of a celestial body, a transit, or a charted object against its computed true bearing, for example a sun azimuth or amplitude observation. Any unexplained change in error signals a deviation shift needing a fresh swing.