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Models and Compass Variation

Geomagnetic field models are used in navigation, geophysical surveys, oil production, and in scientific studies ranging from the upper atmosphere to the Earth's deep interior. BGS has many years' experience in modelling the geomagnetic field on regional and global scales. Compass variation is the angle required to convert between a magnetic bearing and a true or grid bearing.

In Great Britain

Grid Magnetic Angle Calculator

A model of the Earth's magnetic field for the area of Great Britain is updated every year and used to compute magnetic north information for Ordnance Survey map products.


World Magnetic Model (WMM)

The World Magnetic Model (WMM), produced jointly with the US National Oceanographic and Atmospheric Administration's National Geophysical Data Center, is the standard model in UK Ministry of Defence and US Department of Defense navigation and attitude reference systems and is also used widely in civilian navigation systems. The model is also used on marine and aviation charts and is revised every five years. The current model, WMM2015, is valid until 2020.0.

BGS Global Geomagnetic Model

The BGS Global Geomagnetic Model (BGGM) is produced for the oil industry to assist in directional drilling and is revised every year. It includes a significant portion of the crustal magnetic field and large-scale quiet-time magnetospheric field.

International Geomagnetic Reference Field (IGRF)

The International Geomagnetic Reference Field (IGRF) is produced by the International Association of Geomagnetism and Aeronomy (IAGA). BGS plays an active role in its production. Standard geomagnetic coordinates are based on the IGRF.