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Tips and questions relating to the GPS modules from SFE
By jekutin
Does anybody know, if any of the GPS modules give magnetic variation, that is full NMEA RMC message, SiRF does not.
By fll-freak
Military grade GB-GRAM type 1 or 2 and the Micro-Gram modules do! Other than that I am not aware of any other GPS. These are not available to the public. There not that easy for military contractors to get either.

The issue is that to provide magvar, you need to know what the difference between true and magnetic north at every spot on the earth. At the 100 meter scale this is not possible. You would have to account for every metal building, local iron deposits, transmission lines, and other items. Many of these move like a train or car. At the macro scale, the World Magnetic Model (WMM) does provide a reasonable approximation of magvar. WMM accounts for pockets of iron magma in the Earth and the every changing procession of flux. As such, the coefficients to the WMM model need to be updated every 5 years. This is probably why commercial units do not provide it as users are notorious for not updating firmware (
WMM data is NOT available from the GPS satellites).

A solution is to grab the WMM model from the web and code it yourself. It requires lat/long and time as inputs and returns magvar. Time need not be to the second. Something like 2013+6/12 (June) would be adequate.

But just like the Pirate Code, the magvar is just a suggestion, not the truth.