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Re: [gpsd-dev] [gpsd] Altitude in TPV

From: Gary E. Miller
Subject: Re: [gpsd-dev] [gpsd] Altitude in TPV
Date: Thu, 13 Sep 2018 13:18:27 -0700

Yo Peter!

On Thu, 13 Sep 2018 19:45:30 +0000 (GMT)
Peter Liu <address@hidden> wrote:

> I cannot find any definitive answer to whether the Altitude is HAV or
> MSL. Also, which datum is assumed here?

No real definitive answer is possible.  Different GPS report altitude
differently.  gpsd has no way of knowing how your GPS is programmed
and/or configured.  So it just passes on what your GPS told it.

Most GPS will provide MSL calculated internally from ECEF and WGS-84.
But many can, and do, use other datums.

Check your GS doc, and then compare to a known benchmark.

I often put two GPS side by side.  Their lat/lon will agree closely, but
their altitude often differ by 20 feet.  I had one $8k GPS that was off
by 64 feet!

gpsd tries to program u-blox to output ECEF.  Then gpsd uses the ECEF
and its own WGS-84 conversion to get to MSL.  When compared to USGS
benchmarks the agreement is very good.

So, as always: YMMV.

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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