Converting Between Bearings — CADET and CDMVT
To convert between True, Magnetic, and Compass bearings, you add or subtract Variation and Deviation. The mnemonic CADET tells you whether to add or subtract: Compass + Add + Deviation + Equals + Magnetic. Then Magnetic + Add + Variation + Equals + True. When going from Compass to True, ADD easterly errors. When going from True to Compass, SUBTRACT easterly errors.
An alternative mnemonic is 'Cadbury Dairy Milk Very Tasty': Compass → Deviation → Magnetic → Variation → True. Going from left to right (Compass to True), add Easterly errors (and subtract Westerly). Going right to left (True to Compass), subtract Easterly (and add Westerly).
Key Points
- CADET: Compass ADd East for True (Compass → True)
- Cadbury (C) Dairy (D) Milk (M) Very (V) Tasty (T)
- Compass → +Dev → Magnetic → +Var → True (add East errors)
- True → −Var → Magnetic → −Dev → Compass (subtract East errors)
- West errors: do the opposite (subtract when going C→T, add when T→C)
Tip: When in doubt, remember: 'Error East, Compass Least' — if error is East, the compass reading will be LESS than the true bearing. 'Error West, Compass Best' — compass reading is MORE than true.