Candidates most commonly fail on two points: they muddle the direction of conversion (true-to-compass vs compass-to-true), and they misapply the sign of westerly errors — particularly when deviation is involved. Examiners frequently catch candidates out by asking them to work a conversion verbally, without paper, then reversing the direction of the problem.
The Three Bearings
- True (T) — referenced to geographic (true) north.
- Magnetic (M) — referenced to magnetic north; differs from true by variation.
- Compass (C) — what the vessel's compass actually reads; differs from magnetic by deviation.
Variation and deviation are each stated as East or West.
Variation is the angular difference between true north and magnetic north at a given place and time. It is found on the chart compass rose (and corrected for annual change). It is the same for every vessel at that position — it is a property of the Earth's field.
Deviation is the angular difference between magnetic north and the compass north, caused by the vessel's own magnetic influences. It changes with the vessel's heading and must be read from a deviation card for the heading being steered or for which the bearing is taken.
The Mnemonic: TVMDC
True → Variation → Magnetic → Deviation → Compass
- East errors ADD going right (T→C), SUBTRACT going left (C→T).
- West errors SUBTRACT going right (T→C), ADD going left (C→T).
A reliable verbal rule: "Cadbury's Dairy Milk Very Tasty" (C→T: add West, subtract East) for compass-to-true. Reverse the logic for true-to-compass.
Alternatively, many candidates find it cleaner to always work algebraically with a sign convention: East = +, West = −.
Applying T→C: Compass = True − Variation(E) − Deviation(E) Applying C→T: True = Compass + Variation(E) + Deviation(E)
(Substitute negative values for westerly errors and the arithmetic takes care of itself.)
Common Trap
When the total correction exceeds 360° or goes below 0°, add or subtract 360° to bring the result back into the 000–359° range. Forgetting this looks careless in an oral.
Practical Exam Reminder
An examiner may give you a compass bearing and ask for the true bearing to plot, or give you a true course from the chart and ask what you would steer. Know which direction you are converting and apply the correct sign. State your working aloud, step by step — examiners are assessing your method, not just your answer.