AGeoP-24 use of geomagnetic models

What it is

AGeoP-24 is the NATO Allied Geographic Publication that establishes the World Magnetic Model (WMM) as NATO's standard global model of the Earth's magnetic field, "to be installed in instrumentation and to generate data for geographic products". It is promulgated under STANAG 7172: "The agreement of nations to use this publication is recorded in STANAG 7172." AGeoP-24 binds nations rather than suppliers directly, and reaches a contractor only where a national programme or a contract calls for it. The document lists the applications the model covers: navigation, three-dimensional attitude determination, magnetic signature reduction for naval vessels, magnetic detection, and communications equipment. The current edition is Edition A, Version 1, promulgated March 2017; as a first edition, it names nothing it superseded.

Installing the model in a system

Nations agree "to use the World Magnetic Model for any system that needs a global model to be installed in order to maintain interoperability of systems", and to "ensure that the World Magnetic Model is installed in a format that allows the model to be updated easily whenever a new model is produced". The model itself comes from the US National Geospatial-Intelligence Agency (NGA) or the UK Defence Geographic Centre (DGC); a user registers the requirement with either organisation to receive each new model as it is produced. Anyone needing more accurate or real-time geomagnetic information than the World Magnetic Model provides is told to contact the specific country or the model's supplier directly, rather than adapt the standard model. The specification behind the current model is named as U.S. MIL-PRF-89500A, dated 30 October 2015.

Showing magnetic information on a chart or map

Geomagnetic information has to appear on a geographic product in one of five forms, matched to the product type: a Three-Norths diagram for topographic maps, isogonals for air charts and small-scale nautical charts, isogrivs for air charts carrying a navigational grid, a compass rose for nautical charts, or the Magnetic Variation (MAGVAR) object for electronic navigational charts and Additional Military Layers. Whichever form is used, the date of the geomagnetic information has to be given along with the annual rate of change, with the date fixed by the relevant product specification. Isogonals only appear where at least two would be plotted on the chart; otherwise a marginal note gives the approximate declination instead. Isogrivs do the same job for polar charts, using grid north where declination varies too fast for isogonals to stay legible. A compass rose carries the declination in degrees and decimals, the date, and the annual rate of change in minutes per year. The preferred format for the Three-Norths diagram itself is set out in STANAG 3676, not in AGeoP-24.

Choosing the right model for the product

For global products and series, nations use the relevant epoch of the World Magnetic Model. For national or regional products, they may use the latest World Magnetic Model or, where one is available and more accurate, a local magnetic model instead.

Why the model cannot be a one-time install

The document's own glossary explains the reason behind the update requirement: the annual change in magnetic declination "is not constant with respect to time, which is why the model must be replaced periodically". A system or product built around a fixed, unreplaceable model goes out of date on its own, regardless of how accurate it was when installed.

Standards it references

• STANAG 3676Background