DFDL schema for the model file (coefficients file) used by the World Magnetic Model (WMM).
MIL-W-89500 is the military specification for WMM. It can be downloaded from here: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-W/MIL-W-89500_54004/
The format of the model file is described here:
https://www.ngdc.noaa.gov/geomag/WMM/wmmformat.shtml