True, magnetic and compass courses
Compass, magnetic, true
How to think it through
- 1The chart works in true; the helm reads the compass. Two corrections lie between them.
- 2Compass → magnetic with deviation (depends on heading); magnetic → true with variation (from the chart rose).
- 3Going towards true, add East and subtract West (CADET); the other way, do the opposite.
Worked example
Every step says why you do it, which formula you use, the working and the answer.
Compass to True
The helm is steering 005° by compass. What true course are you making?
- Compass course
- 005°C
- Variation
- 6°W
- Deviation (on this heading)
- 5°E
What is the magnetic course?
Why: Magnetic sits halfway: deviation lies between compass and magnetic, variation between magnetic and true.
Working: 005° + 5° = 010°M
Answer: 010°
What is the true course?
Why: From magnetic to true apply the variation: add East, subtract West.
Working: 010° − 6° = 004°T
Answer: 004°