Compass Errors, Deviation and Variation
This domain covers the two corrections standing between a compass reading and a true bearing, and the coefficients that describe how deviation behaves. Expect questions on sign convention, order of application, and where each number comes from.
Variation and deviation
- Variation is the angle between true north and magnetic north at the observer's position. It comes from the chart rose and its annual change, and does not depend on heading.
- Deviation is the angle between magnetic north and compass north, caused by the ship's own permanent and induced magnetism. It changes with heading, cargo, welding and degaussing.
- Each is named east or west according to which side the compass or magnetic north lies from the reference.
Conversion rule
Use T = C + D + V, with east positive and west negative, or C = T - V - D. Apply deviation for the compass heading, interpolating from the deviation card, and variation for the date and position.
Deviation coefficients
Deviation = A + B sin H + C cos H + D sin 2H + E cos 2H, where H is compass heading. A is constant, B and C are semicircular from permanent magnetism, D and E quadrantal from induced soft iron.
Worked example
Compass heading 045 degrees, deviation from the card for 045 degrees is 3 degrees W, variation 7 degrees E. T = 045 + (-3) + (+7) = 049 degrees. Two-step check: compass to magnetic gives 045 - 3 = 042 M; magnetic to true gives 042 + 7 = 049 T. If given true 049 and asked for compass, reverse the signs: 049 - 7 = 042 M, then 042 + 3 = 045 C, taking deviation for the resulting compass heading.
Traps
- Applying deviation taken for the true heading when the card is entered by compass heading; a second iteration may be needed.
- Sign slips: east variation means magnetic is least, so adding it lifts magnetic up to true.
- Ignoring the annual change, or applying it in the wrong direction.
- Reading a deviation card as bearings rather than as errors, or using a card from another cargo or latitude.