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<p>Geo::Compass::Variation works for me, though I suspect that term is more common in the nautical world.</p>
<p>One technical comment on [https://github.com/stevieb9/magnetic-declination/blob/master/lib/Magnetic/Declination.pm|your code] is that it makes more sense for _calculate to return the X,Y,Z magnetic field components and leave the atan2 fiddling to mag_dec and mag_inc. (Maybe mag_dec should have an alias, mag_var.) Since someone might find an interesting use for that information, you might rename _calculate to mag_field and document it as a part of the interface.</p>
<p>BTW, the [https://www.ngdc.noaa.gov/geomag/WMM/data/WMM2015/WMM2015_Report.pdf|technical report] has some test data you could use in your module tests.</p>
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