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[gpsd-dev] ✘wgs_separation()

From: Gary E. Miller
Subject: [gpsd-dev] ✘wgs_separation()
Date: Fri, 19 Jul 2019 15:19:41 -0700

Yo All!

wgs84_separation() now returns EGM2008 data.  This is calculated, as
before, using 10x10 degree data set and doing a bilinear interpolation.
What changed is a move to cm precision data from EGM2008.

There was no documentation on what datum the old data set was.

The resulting precision is one cm at best, from only one meter before.
The tests in show the computed values can still
be 12 meters off.

The doc needs a warning on how bad the computation is.  Doing the
proper computation requires a 450MB dataset.  Probably too big
to include in gpsd, or in any GNSS receiver.  We need to warn the user
to postprocess, with something like GeoidEval, to get the best results.

The 10x10 dataset is 703 integers long.  I guess they could be shorts to
save a bit of space.  I'll prolly run some tests to see if a 5x5 dataset
gives a lot better results.  With a move to shorts that would be
another 1.5kB of data, not bad.

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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