by Bob Thrun » Sep 14, 2013 2:47 pm
The NOAA site uses the IGRF magnetic models that cover the whole earth. It smooths through some local variaations. You should verify its numbers by a surface survey. I have found the IGRF numbers accurate for my caving area. A declination map from an older book, say pre-1950, shows a lot more wiggles in the contours than does a current map.
The code and coefficients used for the IGRF models (They are for five or ten year intervals.) can be downloaded from the NGDC site. The declination calculations are based on the NGDC code.