Prerequisites
- NauticCalc on iPhone (iOS 17.6+)
- Approximate position (GPS or chart)
Step 1: Position
Open Variation in the app. Use current GPS or enter lat/long for your berth.
Step 2: WMM2025 variation
The app computes magnetic variation for place and date. Note degrees and E/W — consistent with your course method.
Step 3: mwK for the leg
For the course chain: rwK ± variation = mwK. Sanity-check the magnitude for your latitude.
Step 4: Do not skip deviation
Variation ≠ deviation. For compass course (MgK) you also need the deviation table.
Read next: Course correction guide · Variation