I think the fog is lifting
Am 11.06.2026 um 19:18 schrieb Greg Troxel via QGIS-User:
Jochem Kail via QGIS-User <[email protected]> writes:
Does your data have an epoch associated with each data point? If no,
does it have an epoch associated with the whole dataset?
The data were all mapped in one month, i.e. one epoch, but several
years ago (2011).
ok, makes sense.
I would now like to compare these river cross-sections with data
recently measured using a DGPS receiver using ETRS89/UTM32N as CRS in
the settings.
sure, I can understand that sort of goal.
Why do you believe that your data is in WGS84 (the ensemble)?
The colleagues used a DGPS receiver and the setting for the CRS was
"WGS84".
That is almost certainly confused. When you use differential GPS, then
the coordinates that come out of the GPS receiver (GNSS?) are not in
WGS84, but they are in the CRS of the reference station.
Your statement is vague, in that "DGPS" implies pseudorange corrections
but *does not specify the kind of reference station or what its CRS is*.
Why do you believe that your data is accurate to on the order of a
mater?
...because it was taken with a DGPS receiver
To realize that accuracy you have to get the CRS right, and you have to
avoid the ensemble.
Do the people who managed/prepared this data understand that 4326 is
an ensemble and that it contains WGS84(TRANSIT)?
No, nor did I. Up to now I thought that WGS84 is non-plate fixed and
therefore a single-fixed geodatic datum, with a plate-fixed geodatic
datum like ETRS89 "moving" relative to WGS84 with the European plate.
WGS84 (specifically EPSG:4326) is an ensemble of a number of datums, all
intended to realize a definition of a system. In this respect it is
like ETRS89 and the various ETRF realizations. The various ensemble
members have names like WGS84(TRANSIT) and WGS84(G2296), the most recent
member. The system definition is more or less congruent with ITRS, and
WGS84 realizations are each very similar to the corresponding (in time)
ITRFxxxx.
You are correct that WGS84 is a global frame with a no-net-rotation
constraint, as opposed to plate-fixed datums.
So, the next step is to say that DGGPS is still vague, what reference
station, and what frame are the reference station coordinates in. Then,
label that data as being in the right frame, and you will likely get a
good transformation.
I believe that it is extremely unlikely that the reference station
coordinates are in any WGS84 realization. There is simply no method to
get accurate coordinates in that frame because carrier phase data from
the GPS reference stations are not published. Every differential system
I have seen or heard about uses either a national/regional datum (e.g. a
NAD83 realization, some ETRF, etc.), or some realization of ITRS,
e.g. ITRF2008.
It is a major problem in GIS that data are mislabeled as being in WGS84
and that it is treated as an interchange format. Both practices are
errors, but also the standard approach.
In summary, I now do suspect, that the data from 2011 were mislabeled as
being WGS84. Based on what I do understand so far, the ITRF reference
frame in 2011 was ITRF2000, which corresponds to the WGS 84 (G1150)
realization. However, most likely the RTK network has been used as
reference stations. Therefore, the geographic coordinates are most
likely based on the ETRS89 reference system (not WGS84) and given that
the data have been mapped in 2011, based on the ETRF realization ETRF2000.
No I am still struggeling to find a way to do a time-dependent
transformation in QGIS from ETRS89/ETRF2000 to the recent ETRS89/ETRF2020.
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--
sent from my R2 unit
-------------------------------------------------------
Jochem Kail
Dr. rer. nat.
University of Duisburg-Essen
Faculty of Biology
Department of Aquatic Ecology
Universitätsstrasse 5
D-45141 Essen
Germany
Room S05T03B08
Tel: +49.201.183.3046
Tel: +49.160.96228374
Fax: +49.201.183.2179
[email protected]
https://www.uni-due.de/aquatische_oekologie/mitarbeiter/jochem_kail.php
Important note: The university server does not accept old Microsoft Office
files (e.g. *.doc) but only the recent version (e.g. *.docx)!
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