Module 9 - Compass
Converting Between Bearings — CADET and CDMVT
What you will learn
Convert courses and bearings both ways through CDMVT, retain the magnetic intermediate and check signs, heading-dependent deviation and 000°/360° wrapping.
A mnemonic is only a direction check. Reliable coastal work labels every stage, attaches a sign to each error and shows the magnetic intermediate so another navigator can audit it.
Write Compass—Deviation—Magnetic—Variation—True. In the signed convention used here, east is positive and west negative when moving C→T: Magnetic = Compass + deviation, then True = Magnetic + variation. To work T→C, reverse each operation: Magnetic = True − variation, then Compass = Magnetic − deviation.
For 180°C with 2°E deviation and 3°W variation, write +2° and −3°: 180 + 2 = 182°M; 182 − 3 = 179°T. For 250°T with 4°W variation and 2°E deviation: 250 − (−4) = 254°M; 254 − (+2) = 252°C.
CADET means 'Compass to True, Add East' as a direction reminder, not a substitute for working. Select deviation for the intended compass heading. If the card is coarse and the conversion changes which heading bracket applies, recheck or iterate according to the chartwork method. Wrap 361° to 001° and −2° to 358° before presenting the answer.
Worked example
Convert a planned 250°T course to compass using variation 4°W and residual deviation 2°E on the intended heading.
- 1Write signed errors in the C→T convention: variation −4°, deviation +2°.
- 2Reverse the variation step: 250°T − (−4°) = 254°M.
- 3Reverse the deviation step: 254°M − (+2°) = 252°C.
Sense check: The 4°W variation makes magnetic greater than true; the 2°E deviation then makes compass 2° less than magnetic.
Audit a plausible direct answer
A passage-plan worksheet jumps from 250°T to 248°C by combining the lettered errors without showing magnetic.
1. Restore the references
Write T—V—M—D—C because the conversion is running from true to compass.
2. Reverse signed operations
Subtract −4° variation to obtain 254°M, then subtract +2° deviation to obtain 252°C.
3. Check the card and range
Confirm that 2°E is the residual deviation for the resulting compass heading and normalise the answer to 000°–359°.
Sense check: The magnetic intermediate exposes both a sign error and a correction taken from the wrong source.
Common mistake or limitation
- 'Add east' reverses when working from true to compass. Without writing the direction, the same mnemonic can produce the opposite answer.
- Compass Least/Best is a final sense check, not a method for skipping the magnetic intermediate or heading-specific deviation.
Recap
- Write CDMVT, direction and signed errors before arithmetic.
- Show the magnetic intermediate and suffix every answer.
- Recheck deviation, rounding and 360° wrapping before using the course.
Optional quick check
Section 4 of 7
Convert 120°C with 3°E deviation and 2°W variation to true using signed CDMVT working.
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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