Variation (declination) is the angle between geographic north and magnetic north — it changes with place and time. Every piece of course work needs the current value for your area. This guide shows how to query it in NauticCalc from the WMM2025 model and carry it into the course chain.
Prerequisites
- NauticCalc on the iPhone (iOS 17.6+)
- Approximate position (GPS or chart)
- Clarity about your course method (how you apply east/west or plus/minus)
Step 1: Position and area
Open Variation in the app. Use the current GPS position or enter latitude/longitude for your berth. For planning the centre of the area is enough; on long legs the value can change noticeably.
Step 2: Variation from WMM2025
The app computes the magnetic variation for place and date from the World Magnetic Model WMM2025. Note degrees and east/west — consistent with your course method. The model is valid for a fixed period; watch for later model changes.
Step 3: mwK for the leg
For the course chain: mwK = rwK ± variation. Sanity-check the magnitude — depending on latitude and area it is often only a few degrees, but in some regions considerably more.
Step 4: Do not forget deviation
Variation ≠ deviation. For the steerable compass course (MgK) you additionally need your boat's deviation table. Only both together give the course on the ship's compass.
Common mistakes
- Using an outdated year — mind the model change
- Swapping east and west
- Confusing variation with deviation
- Taking the area centre for a very long leg even though the value changes
Read next: Course correction guide · Variation feature