Magnetic declination

Model: WMM2025 (degree 12) or IGRF-14 (degree 13) spherical-harmonic main field with linear secular variation, on the WGS 84 ellipsoid

Show your work

1. Model and epoch

   the field the model gives for this place at this date

   wmm2025 at 2,026.712 = 51,083 nT total

2. Horizontal components

   X north and Y east, the part of the field a compass follows

   X 20,575 nT, Y 2,770 nT = 20,760 nT horizontal

3. Declination

   D = atan2(Y, X)

   atan2(2,770 nT, 20,575 nT) = 7.67°

The same steps an agent gets from the MCP server with explain: true.

Accuracy: Matches all 100 NCEI WMM2025 test values (declination and inclination to their printed 0.01°, intensities within 0.001 nT). The model itself is good to about 0.3° of declination away from the poles; local crustal anomalies of several degrees are not modeled.

When to use this: Use this whenever a magnetic direction meets a true one: setting a compass or a heading indicator, converting a runway or a chart bearing, checking the variation for a flight plan or a survey, or seeing how strong and how steep the field is for a magnetometer. It runs WMM2025 or IGRF-14 back to 1900.

Limitations: A model is a smooth global field: local magnetic anomalies, iron structures, and vehicle deviation move a compass off it, and the model carries its own published uncertainty, which grows toward the poles. Declination changes year to year, so the date matters, and a model has a validity window that this tool enforces rather than extrapolating past.

Worked example: Boulder, Colorado on September 18, 2026. Source: NCEI WMM2025 synthesis; checked against the published WMM2025 test values (WMM2025_TestValues.txt).

Review: Not yet independently reviewed by a geodesist.

Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.1, core 0.1.0. See this tool in the verification report.

Changes

• 2026-09-23, fixed: Identifiers no longer show digit grouping: the EPSG code in SPCS83 forward and zone lookup reads 32129, not 32,129, and the decimal year in magnetic declination reads 2026.71, not 2,026.71 (all version 1.0.1). SPCS83 forward in US survey feet also gave the deprecated-unit warning (LEGACY_UNIT) twice, once for easting and once for northing; every tool now gives it once per result. Values are unchanged. Changelog
• 2026-09-23, fixed: Plain-number results in a fixed unit now show that unit: the battery current reads 9.7 A where it read 9.7, and decibels, gigabytes, pulses per square meter, grams per kilogram, nautical miles per gallon, hours, nanoteslas, and minutes now carry their units too. The numbers themselves do not change. Each tool's patch version went up. Changelog
• 2026-09-18, changed: The first six tools passed the stable verification bar: a derivation, an independent worked example, invariant and differential tests, and at least 20 golden vectors each. Changelog

Checked against: 29 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• The US/UK World Magnetic Model for 2025-2030: Technical Report, NOAA National Centers for Environmental Information and British Geological Survey, WMM2025, NESDIS/NCEI (December 2024). Section 1.2 (equations 1-25: synthesis, secular variation, and elements) and Section 3 (uncertainty, blackout and caution zones).
• International Geomagnetic Reference Field: the fourteenth generation, International Association of Geomagnetism and Aeronomy (IAGA), Working Group V-MOD, IGRF-14 (igrf14coeffs.txt). Coefficient table, 1900.0-2025.0 main field and 2025-2030 secular variation.

Terms

WGS: World Geodetic System

The US Department of Defense global reference frame and ellipsoid that GPS uses; the current version is WGS 84. Source: Department of Defense World Geodetic System 1984