[Qgis-user] Do GPX files contain CRS information?
Greg Troxel
gdt at lexort.com
Fri Mar 5 13:48:18 PST 2021
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.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 194 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20210305/936710c4/attachment.sig>
More information about the Qgis-user
mailing list