The World Magnetic Model (WMM) is the standard model used by the United Kingdom and the United States governments, including the U.S. Federal Aviation Administration and the U.S. Department of Defense, as well as organizations with an international remit such as the NATO.
The model, which is updated every five years, comprises a series of magnetic field maps that track changes in the magnetic field, such as the spot at which compass needles point in the northern hemisphere. To ensure pinpoint accuracy, it is crucial that the shifts in magnetic north, which are caused by flow of the liquid iron in the outer core of the Earth, are taken into account by global positioning systems (GPS) that are relied upon across the world.
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