What this is about
Whether you are studying for the RYA Day Skipper, the Yachtmaster theory or a European licence, course conversion comes up in almost every navigation exercise: converting between the course your compass shows and the course you draw on the chart. It sounds dry – once the scheme clicks, it is pure routine.
This article covers the scheme, the sign rules and two fully worked examples: from the compass to the chart, and from the chart to the compass.
Note: Abbreviations differ between training schemes. We use common English terms and, where helpful, the German abbreviations shown in NauticCalc (MgK, mwK, rwK, KdW, KüG).
The courses at a glance
| Course | Meaning | Reference |
|---|---|---|
| Compass course (C) | what your steering compass shows | compass north |
| Magnetic course (M) | compass course corrected for deviation | magnetic north |
| True course (T) | magnetic course corrected for variation | true (geographic) north, no wind or current |
| Course through the water (CTW) | true course plus leeway | water |
| Course over ground (COG) | course through the water plus tidal stream | seabed |
Each step adds one correction:
| From | Correction | Result |
|---|---|---|
| C | + deviation | = M |
| M | + variation | = T |
| T | + leeway | = CTW |
| CTW | + tidal stream | = COG |
- Deviation – error caused by iron and electronics on board. It depends on the heading and comes from your boat's deviation card.
- Variation – angle between magnetic and true north, printed on the chart's compass rose. → What is magnetic variation?
- Leeway – how far the wind pushes the boat off its heading.
- Tidal stream / current – found with the tidal stream triangle.
Key point: wind and current are applied after the true course. The true course is the course without drift; the line you actually make good over the ground is the course over ground.
Signs: the most common mistake
- East = plus, West = minus – for deviation and variation.
- Wind from starboard pushes the boat to port, the course number gets smaller: leeway negative.
- Wind from port pushes the boat to starboard, the course number gets larger: leeway positive.
Working backwards (from the chart to the compass), you invert every sign: plus becomes minus. The classic mnemonic for true → magnetic is "East is least, West is best": subtract easterly variation, add westerly.
Updating variation from the chart
The compass rose might say 3°10' W 2020 (9' E): in 2020 the variation was 3°10' West, and it changes by 9' eastward each year.
For 2026: 6 years × 9' = 54' east, so 3°10' W − 54' = 2°16' W. Rounded, variation = −2°.
NauticCalc skips this step: the app calculates variation for your position and date directly from the World Magnetic Model 2025.
Example 1: from the compass to the chart
Given: You steer compass course 085°. The deviation card gives +10° at 085°. Variation is 2° W. The wind blows from 150°, leeway is 7°.
| C | Dev | M | Var | T | Leeway | CTW |
|---|---|---|---|---|---|---|
| 085° | +10° | 095° | −2° | 093° | −7° | 086° |
Why leeway −7°? The boat heads about 093°, the wind comes from 150°, i.e. from starboard. The boat is pushed to port, so the course number drops.
Result: The course through the water is 086° – that is the line you draw on the chart. If there is a tidal stream, the stream triangle then gives you the course over ground.
Example 2: from the chart to the compass
Given: You want to make course through the water 132°. The wind blows from 060°, leeway is 8°. Variation is 2° W, deviation around 126° is +6° according to the card.
Working backwards means inverting every sign.
| CTW | − Leeway | T | − Var | M | − Dev | C |
|---|---|---|---|---|---|---|
| 132° | − (+8°) | 124° | − (−2°) | 126° | − (+6°) | 120° |
Why leeway +8°? Heading about 124°, the wind from 060° comes from port. The boat is pushed to starboard, so leeway is positive – and is subtracted when working backwards.
Result: Steer compass course 120°.
Check forwards: 120° + 6° = 126° · 126° − 2° = 124° · 124° + 8° = 132° ✓
Tip on deviation when working backwards: The deviation card is indexed by compass course – the very value you are looking for. In practice you use the value at the magnetic course. If deviation changes quickly in that range, confirm the result with the forward check.
The five most common mistakes
| Mistake | Consequence |
|---|---|
| Forgetting the sign for West: 2° W is −2°, not +2° | course wrong by twice the variation |
| Not inverting the signs when working backwards | the correction is doubled instead of removed |
| Leeway applied in the wrong direction | course wrong by twice the leeway |
| Variation not updated to the current year | systematic error over the whole passage |
| Deviation read for the wrong heading | several degrees off, depending on the card |
Checking with NauticCalc
For the exam you must master the calculation by hand – apps are not allowed in the exam itself. For checking your practice exercises, and later on board, course conversion in NauticCalc helps:
- from compass course to course over ground, forwards and backwards
- your own deviation card for each boat
- current variation from the World Magnetic Model 2025, no chart arithmetic
- leeway and tidal stream in the same calculation
The calculators work without internet – including at sea with no signal. → Sailing without internet
FAQ
What is the difference between variation and deviation? Variation depends on location (the Earth's magnetic field) and is printed on the chart. Deviation depends on the boat and its heading and comes from the deviation card.
What is compass error? The sum of deviation and variation. It takes you straight from the compass course to the true course.
How do I estimate leeway? In exam questions it is usually given. On board, measure the angle between the boat's centreline and its wake, or use experience values for your boat.
This guide does not replace a navigation course. Check the conventions of your course materials.
Read more: Course conversion in the app · The course chain rwK, mwK, MgK · Tidal stream triangle · Download