API & MCP

Everything a person can do on Ohmforge Magnetic Declination (WMM2025), a program or AI agent can do with the same inputs and get the identical result.

HTTP API

POST https://ohmforgemagneticdeclination.usefulweb.app/api/v1/calculate – run Ohmforge Magnetic Declination (WMM2025)
GET https://ohmforgemagneticdeclination.usefulweb.app/api/v1/tool – inputs, output schema, examples
OpenAPI 3.1: /openapi.json · /openapi.yaml
Errors: application/problem+json; 422 includes per-field errors.

Ohmforge Magnetic Declination (WMM2025)

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.

Inputs of Ohmforge Magnetic Declination (WMM2025)
Field	Type	Default	Description
`latitude`	number	52.52	Latitude. North positive, south negative (WGS84).
`longitude`	number	13.405	Longitude. East positive, west negative.
`height`	number	0	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.

Example request

Berlin, 8 November 2026

curl -X POST https://ohmforgemagneticdeclination.usefulweb.app/api/v1/calculate -H 'content-type: application/json' -d '{"latitude":52.52,"longitude":13.405,"height":34,"date":"2026-11-08"}'

Every result contains

inputs (normalised, defaults applied), summary, result (validated against the published schema), units, assumptions, sources, warnings, computedAt and links.

MCP (Model Context Protocol)

Streamable HTTP endpoint: https://ohmforgemagneticdeclination.usefulweb.app/mcp – protocol revisions 2026-07-28 (stateless) and 2025-11-25 / 2025-06-18 / 2025-03-26 (initialize handshake).
Tools: ohmforge_magnetic_declination
Local stdio connector: node src/mcp-stdio.ts --tool=ohmforge-magnetic-declination

{
  "mcpServers": {
    "ohmforge-magnetic-declination": { "type": "http", "url": "https://ohmforgemagneticdeclination.usefulweb.app/mcp" }
  }
}