What magnetic variation actually is
When you read a course from the chart and steer by compass, you quietly assume the compass shows true north. In reality, Earth’s magnetic field in your area points slightly differently from the chart’s true north. That difference is magnetic variation (German: Missweisung).
Variation is location- and time-dependent. It is described by the World Magnetic Model (WMM) and changes slowly over the years. In practice you need the current value for your harbour or cruising grounds — not an old note from a previous season.
Variation and deviation: don’t mix them up
Two terms are often spoken together but mean different things:
- Variation: the angle between magnetic north and true north — a property of Earth’s field.
- Deviation: the extra error your boat adds — iron, engine, electronics, speakers near the compass.
You correct deviation with a deviation table (compass compensation). You apply variation from the model or chart in the course chain. Mixing the two means double-counting or missing a correction.
The course chain in brief
To go from chart course to compass course you typically follow:
- rwK — true course (from the chart)
- mwK — variation-corrected course: rwK ± variation (East subtract, West add — check your training mnemonic)
- MgK — compass course: mwK ± deviation
- Plus allowances for wind and current when you need the course over ground (COG)
We explain rwK, mwK, and MgK in rwK, mwK, and MgK: the course chain. Wind, current, and the full workflow live under course correction.
Why it matters at sea
A few degrees of error grows quickly over distance. At 10 nautical miles and 5° off, you are already well wide — enough to miss a buoy or get uncomfortable in shallow water.
Traditionally you work with triangle, ruler, and tables. NauticCalc handles variation (WMM), deviation, and the course chain on iPhone — without a network once the app is installed. See offline navigation for details.
Where to get variation — practical sources
- Chart / year of edition: often in the compass rose or legend
- WMM / app: current value for your latitude and longitude
- GPS course vs. compass: rough plausibility only — not a substitute for proper navigation
In NauticCalc, variation for your position comes from the magnetic model. You can feed it straight into course correction from the variation tool — no retyping tables.
Typical mistakes
- Wrong sign (East/West)
- Outdated variation from an old chart
- Deviation applied twice — or not at all
- Current forgotten although set and drift are known
Bottom line
Variation is not exam trivia — it is part of safe coastal and offshore sailing. If you keep rwK, mwK, and MgK separate and use current values, you steer more accurately — even when mobile data is gone.
Read next: Variation in NauticCalc · Course correction · Offline at sea