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The model in figure 28 below is a combined effort between NOAA and the BGS called the World Magnetic Model which is updated every 5 years. This is a lower order model, as is the International Geomagnetic Reference Field produced by IAGA but these are freely accessible over the internet whereas the higher order models require an annual license.
The higher order world models (BGGM and HDGM) are considered to be better than 1 degree (99% confidence) at most latitudes. This may be less true at higher latitudes above 60 0 but at these latitudes, IFR techniques are frequently used.
Figure 29: Typical magnetic observatories.
Figure 28: Main field declination Jan 2010.
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