Module 9 - Compass
Converting Between Bearings — CADET and CDMVT
What you will learn
Convert courses and bearings between compass, magnetic and true with signed, labelled steps and a final direction-and-magnitude check.
CDMVT keeps the order visible: Compass—Deviation—Magnetic—Variation—True. This course treats east errors as positive and west errors as negative when working from compass to true; reverse the operation from true to compass.
Write C—D—M—V—T on the page and attach a sign to each error: east positive, west negative. From compass to true, Magnetic = Compass + Deviation, then True = Magnetic + Variation. From true to compass, reverse each operation: Magnetic = True − Variation, then Compass = Magnetic − Deviation.
Example C→T: 180°C, deviation 2°E and variation 3°W. Write +2° and −3°: 180 + 2 = 182°M; 182 − 3 = 179°T. Example T→C: 250°T, variation 4°W and deviation 2°E. 250 − (−4) = 254°M; 254 − (+2) = 252°C.
CADET is a direction reminder—Compass to True, Add East—not permission to skip labels. Keep intermediate magnetic working, use the deviation for the relevant compass heading, wrap results through 000°/360° and sense-check whether an east error made the compass value less than the corresponding true value.
Worked example
Convert 250°T to compass with variation 4°W and deviation 2°E.
- 1Write signed errors: variation −4°, deviation +2°.
- 2True to Magnetic reverses the variation step: 250 − (−4) = 254°M.
- 3Magnetic to Compass reverses the deviation step: 254 − (+2) = 252°C.
Sense check: The 4°W variation first makes magnetic numerically greater than true; the 2°E deviation then makes compass 2° less than magnetic.
Audit a plausible but wrong answer
A worksheet jumps directly from 250°T to 248°C by adding the two lettered errors without showing magnetic.
1. Restore the sequence
Write T—V—M—D—C for the reverse direction.
2. Attach signs
Variation 4°W is −4° and deviation 2°E is +2° in the C→T convention.
3. Reverse operations
Subtract each signed error to obtain 254°M then 252°C.
Sense check: Intermediate magnetic working exposes whether variation and deviation were applied in the correct places.
Common mistake or limitation
- Adding the letters E/W by rote without writing the conversion direction makes the same mnemonic produce opposite answers.
- Deviation is selected for the intended compass heading, so a large conversion may require a checked iteration rather than a careless card lookup.
Recap
- Write CDMVT and signed errors before arithmetic.
- Show the magnetic intermediate and suffix every answer.
- Use Compass Least/Best only as a final sense check, not the working method.
Optional quick check
Section 4 of 5
Using east-positive signed working, convert 120°C with 3°E deviation and 2°W variation to true.
Continue studying Compass
This public lesson is part of Revision Module 9. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.