Definition
Produced jointly by the British Geological Survey and the US NOAA National Centers for Environmental Information, the WMM is a spherical harmonic representation of the main geomagnetic field, released on a five year cycle with annual change coefficients. It is the source of the variation figure computed by chartplotters, autopilots and electronic compasses, and specified for defence use. Its stated accuracy degrades near the poles and it cannot represent local magnetic anomalies caused by geology.
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