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Magnetic Declination in map.army — True vs Magnetic North
Tactical map work routinely needs the difference between true north (geographic) and magnetic north (where a compass needle points). map.army computes that difference — the magnetic declination — at every point on the map, lets you choose which reference your bearings are reported against, and reorients the view to either north as needed.
Magnetic declination model
The magnetic declination at any point on Earth changes slowly over time as the geomagnetic field drifts. map.army uses the World Magnetic Model (WMM) maintained by NOAA / NCEI and the UK Defence Geographic Centre. The current model is:
| Model edition | Valid for | Notes |
|---|---|---|
| WMM2025 | 2025-01-01 to 2029-12-31 | Current model used by the application. |
WMM is the international standard reference used by NATO, US DoD, FAA, IHO, and most national militaries. The model is updated every five years; we update map.army when the next edition is published.
True vs. magnetic bearings
Bearings displayed by the Measure Distance tool and the symbol modifiers can be reported as true (relative to geographic north) or magnetic (relative to local magnetic north). The switch lives under Options → General → Angular Unit: pick degrees, milliradian (NATO), and the reference (true or magnetic).
The magnetic-declination correction is computed for the geometric centre of the measured segment using the WMM model — for short segments (kilometres) this is effectively a single value; for long-range segments the declination is interpolated.
GM angle (grid-magnetic angle)
When a coordinate grid is enabled, the angle between grid north (the local grid’s north meridian) and magnetic north is the GM angle. It is shown on the coordinate-grid display when the grid is active — see Map → Coordinate Grid.
GM angle = (declination) ± (grid convergence). In areas with significant grid convergence (high latitudes, off-meridian UTM zones), the GM angle can differ noticeably from raw magnetic declination — military planning typically uses GM angle, not declination alone.
Orient to north
To reorient the map view:
- Snap to true north — click the Compass button at the bottom-left (this resets any rotation introduced by 3D view or pinch-rotate touch gestures).
- In 3D view, press N on the keyboard to face north — see 3D Map View — Compass.
- Rotation to magnetic north is not a one-click toggle; for tactical overlays we recommend keeping the view in true north and using the magnetic-declination / GM-angle annotation in the coordinate grid for compass-relative readings.