Why there are several “courses”
On the chart you plan in true (geographic north). At the wheel you read the compass — magnetic north, plus your boat’s deviation. Corrections sit in between. The course chain orders those steps so you get from the planned chart course to the compass course (or the other way around).
rwK — true course
rwK is the course you read or plot on the chart: direction over the ground relative to true north. It is the basis of most passage planning.
Example: you want to go from waypoint A to B — rwK is the direction of the line on the chart (variation not applied yet).
mwK — variation-corrected course
From rwK to magnetic direction you apply variation:
mwK = rwK ± variation
A common mnemonic: East least, West best — for easterly variation subtract, for westerly add (always verify the sign on your chart or model).
We cover variation in What is magnetic variation?. The calculator is under Variation.
MgK — compass course
MgK is the course you steer on the compass before adding wind and current as allowances:
MgK = mwK ± deviation
Deviation is boat-specific. Without compensation it can change a lot with heading — hence the deviation table.
From MgK to course over ground (COG)
Wind and current shift your track over the ground. You allow for them:
- Wind allowance (lee): leeway from wind
- Current allowance: set and drift
COG (course over ground) is what you see on the chart again. Course correction in NauticCalc ties rwK, variation, deviation, wind, and current in one workflow — including the current triangle when set and drift are known.
Working backwards: from compass to chart
After a bearing or compass reading you may want the chart picture:
- Read MgK
- Apply deviation and variation backwards → rwK
- Apply current/wind → COG
Errors add up both ways. A consistent app helps keep signs straight.
Example (simplified)
| Step | Value | Note |
|---|---|---|
| rwK from chart | 095° | planned direction |
| Variation | 3° E | East least → mwK = 092° |
| Deviation | 2° W | MgK = 094° |
| Current BS | 4° | steering/COG depends on task |
Exact sign logic depends on the problem and your textbook — use one consistent method and check plausibility (land in sight, GPS roughly).
Offline on the bridge
The course chain and allowances are core calculators in NauticCalc. They run without internet — important offshore or in coastal dead zones. What else works offline (tides, logbook, GPS) and what needs a network or your own API keys is in Sailing without internet and Offline navigation.
Typical exam and practical traps
- Variation and deviation swapped
- Current treated as a “course” instead of an allowance
- rwK and COG confused when planning ETA
- Old table values instead of current WMM
Bottom line
rwK, mwK, and MgK are not random abbreviations — they are the links from chart to steering compass. Master the chain and you can add wind and current deliberately — and run the math quickly and traceably with NauticCalc at sea.
Read next: Course correction step by step · Current correction · Variation · Download the app