Chapter 6: Compasses

Variation and deviation

Variation is the angle between true and magnetic north at a place; it is printed in the chart's compass rose with its annual change and is independent of the vessel. Deviation is caused by the ship's own permanent and induced magnetism and changes with the vessel's heading; it is tabulated on a deviation card produced by swinging ship. Both are named East or West.

The TVMDC chain

Working from True to Compass you add westerly errors: 'Can Dead Men Vote Twice At Elections' — Compass, Deviation, Magnetic, Variation, True, Add East (going from compass to true). Going the other way, from True down to Compass, add westerly errors and subtract easterly ones. Compass error is the algebraic combination of variation and deviation and is applied in one step when convenient.

• Compass → True: add East, subtract West
• True → Compass: add West, subtract East
• Deviation is taken from the card for the compass heading, not the true heading

Gyro and modern sensors

The gyrocompass seeks true north mechanically and is unaffected by magnetism, but suffers speed, latitude and ballistic errors and must be checked daily by azimuth or amplitude observation. Gyro error is named high or low: if the gyro reading exceeds the true bearing, the gyro is high and the error is subtracted. Fluxgate and satellite compasses (GNSS heading) are common on small craft; all require an independent cross-check.