# Ohmforge Magnetic Declination (WMM2025)

> Magnetic declination, inclination and field strength for any place and date (2025–2030). Computes the Earth's main magnetic field with the World Magnetic Model WMM2025 of NOAA and the British Geological Survey: declination (how far a compass needle points away from true north), inclination, horizontal and total field strength, the yearly change and the grid variation near the poles. Optionally converts a compass bearing to a true bearing. Same model as the compass in Ohmforge's Pocket instruments.

Part of usefulweb.app (https://usefulweb.app/). Part of [Ohmforge](https://ohmforge.usefulweb.app/) (ownerApp: ohmforge). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://ohmforgemagneticdeclination.usefulweb.app/ (interactive tool at https://ohmforgemagneticdeclination.usefulweb.app/tool)
- JSON API: POST https://ohmforgemagneticdeclination.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://ohmforgemagneticdeclination.usefulweb.app/api/v1/tool)
- OpenAPI: https://ohmforgemagneticdeclination.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://ohmforgemagneticdeclination.usefulweb.app/mcp - tool `ohmforge_magnetic_declination`

## Inputs
- latitude (number): Latitude - North positive, south negative (WGS84).
- longitude (number): Longitude - East positive, west negative.
- height (number): Height above sea level - Treated as height above the WGS84 ellipsoid; the difference (up to about 100 m) does not matter here.
- date (text): Date - YYYY-MM-DD or a decimal year such as 2027.5, between 2025.0 and 2030.0. Empty = today.
- bearing (number): Compass bearing to convert (optional) - A magnetic bearing read from a compass; the result adds the declination to get the true bearing.

## Method
- The World Magnetic Model WMM2025 (degree and order 12, epoch 2025.0) is evaluated as a spherical-harmonic series: geodetic WGS84 coordinates are converted to geocentric spherical ones, Schmidt semi-normalised Legendre functions are built by recursion, the Gauss coefficients are moved to the date with their yearly change, and the field vector is rotated back to the local north/east/down frame.
- Declination D = atan2(east, north), inclination I = atan2(down, horizontal); their yearly changes follow from the yearly change of the components. Near the poles (|latitude| ≥ 55°) the grid variation (declination relative to grid north of a polar stereographic map) is also given.
- The model reproduces all 100 points of the official WMM2025 test value list to the printed precision (0.01° and 10⁻⁶ nT). A true bearing is the magnetic bearing plus the declination.
- Where the horizontal field is below 6000 nT a compass becomes sluggish (caution zone); below 2000 nT the WMM report calls the declination unreliable (blackout zone).

## Limits
- The WMM describes the main field from the Earth's core only. Local magnetism (rocks, steel, cars, power lines, the phone itself) and magnetic storms can turn a compass by several degrees; do not rely on a phone compass for navigation where safety matters.