[gpsd-dev] ✘magnetic variation

[Gary E. Miller]

Yo All!

FYI:

I added magnetic variation support to the NMEA drivers and the TPV JSON.

Sadly, most GPS do not supply magnetic variation.  I looked into calculating
it like gpsd calculates the geoid separation.  Make a table for the
entire earth at 5x5 degree resolution, and interpolate.

Turns out I can not find a source of magnetic variation that works
near the poles.  So now I know why GNSS receivers typically do not
try to calculate the magnetic variation.

I found two that are easy to use:

https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml

MagneticField from https://geographiclib.sourceforge.io/

Both fail the same way at the poles.  

For example.  If I am at the North Pole (90N), then the longitude does
not affect my location.  Yet the calculators gives me wildly different
results based on the longitude?

    90N 0W 2.79

    90N 90W 82.20

    90N 180W 177.21

    90N 90E 92.79

    90N 180E 177.21

Similarly for the South Pole:

What a mess...  Anyone know a working magnetic variation calculator?

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

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