[Qgis-user] Do GPX files contain CRS information?

[Greg Troxel]

Nyall Dawson <nyall.dawson at gmail.com> writes:

> On Fri, 5 Mar 2021 at 14:59, jeremy benwell <jeremybenwell at gmail.com> wrote:
>>
>> I was wondering if GPX files contain CRS information? I saved some
>> waypoints in my garmin gps and then used GPSBabel to upload those
>> waypoints to my computer and create a GPX file. The map datum on my
>> gps was set to GDA 94. Does my GPX file contain the CRS that my GPS
>> was set to when I saved the waypoints (i.e. GDA 94) and if so do I
>> need to make sure my QGIS project CRS is set to GDA 94 also?

The format spec is available:
  https://www.topografix.com/gpx.asp
  https://www.topografix.com/GPX/1/1/

The spec is clear that the datum is WGS84.

For elevation, I read the spec as saying that the datum is "WGS84
orthometric height", meaning that one takes WGS84 ellipsoidal height and
uses EGM2008 to get a height that is sort of "above sea level".  The
notion that the height is ellipsoidal height is to me unreasonable.  I
have observe garmin units using the "orthometric height" notion; my
geoid separation is around -30m and this can be checked.  I have an open
question to the GPX spec group about this.

> No, they don't store CRS. Definitely make sure you set the crs to
> GDA94 (but only when the gps was explicitly set to this datum!!). I'd
> suggest re-saving the gpx to a format like gpkg which can store the
> gda94 crs correctly, so that the dataset will open in future with the
> correct CRS and you won't run into issues later.
> (GPX files are just text files -- you can open to verify this!)

I am pretty sure that Garmin units use the configured datum only for
display, and that storage (which is actually GPX, in non-ancient units))
and GPX is still in WGS84.  If not, they are IMHO buggy.  But I haven't
set a datum in mine since I used NAD27 when displaying in UTM to find
points with a ruler on older topo maps!


I would suggest to the Jeremy to understand the delta from "WGS84" to
GDA94.  I'm not a geodesy.expert.au, but my impresssion is that it's
only a few meters and that it is therefore unlikely that points from a
Garmin unit have errors that are small enough to notice that.  I have
not been able to notice the NAD83(2011)/WGS84(G1762) shift (about a
meter) with L1-only navigation solution GPS.  I can resolve it very
clearly with dual-frequency multi-constellation RTK.


Also, Jeremy should be aware of the "null transform" issue with WGS84.
WGS84 will be considered equivalent to a number of datums.  For me, it
is mapped without a shift to NAD83 and also to ITRF2014.   So if I
display data that's actually in ITRF2014 and also data labeled WGS84,
and change my project CRS from ITRF2014 to NAD83(2011), the relative
position of points change.

Despite "GPX is WGS84", if the GPS receiver was receiving differential
corrections, either locally or via SBAS such as WAAS, then the output
coordinates are no longer in WGS84 and are instead in the differential
system's frame.  WAAS is I believe in something like ITRF2005, but it's
very hard to figure that out precisely.  (My understanding is that at
least most of Australia currently has no available SBAS, but almost all
measurements made in the US with navigation-grade equipment are with
WAAS.)

The best approach is to stop using WGS84 as a CRS, and label the data
with a more precise CRS.    Even for L1-only GPS with no corrections,
today you get WGS84(G1762), which should not be subject to the "null
transform" problem.    This is what Nyall suggested, far more concisely
then I did :-)

Greg


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