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

[Greg Troxel]

Nicolas Cadieux <njacadieux.gitlab at gmail.com> writes:

>> 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.  
>
> If the standard says orthometric height, it means that it takes the
> ellipsoïdal height and then applies the geiod model (in this case
> EGM2008 or Earth gravitational model 2008).  This is the height where
> the average sea level would be given the local gravity on land.
> Orthometric height is the geiod height or the height above the average
> sea level.

Agreed but what it says is:

  <xsd:element name="ele" type="xsd:decimal" minOccurs="0">
    <xsd:annotation>
      <xsd:documentation> Elevation (in meters) of the point. </xsd:documentation>
    </xsd:annotation>
  </xsd:element>

To me, "elevation" always means some kind of orthometric height.  I have
never heard anyone call an ellipsoidal height elevation.  Given the
notion of WGS84 in GPX, and that WGS84 defines orthometric height, I
find this unambiguous -- but not comfortably so.

>> 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.
>
> In North American, most devices do not make a difference between Nad83
> (revised models) and WGS84 (revised models).  I imagine this is
> probably the case with GDA94, specially if GDA94 was identical to
> WGS84 original in the beginning (i’am not sure this is the case, I
> really don’t know here).

Agreed.  I think what you are saying is that when one asks a device to
datum transform from WGS84 to NAD83 it will use null transform.

>> 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.)
>
> Weird... I would expect the coordinates to be a simple corrections of
> whatever version of WGS84 is currently in use...

I expected that too.  It seems not to be though.

The reference stations that generate the coordinates don't have a way to
get precise WGS84(G1762) coordinates.  And, GLONASS/Galileo/BeiDou don't
use WGS84.  It all amounts to a bunch of frames which are for practical
purposes equivalent (ITRF2008 is a good overall description today, I
think, but that and ITRF2014 are really close).

My theory is that until you get to RTK, you just aren't going to get
sub-meter.  So worrying about which modern (>= 2005) flavor of
WGSF84/ITRF/IGS is academic.

If you can find a clear statement of what frame any SBAS uses, I'd love
to see a URL/pointer.
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