Dear Greg,
many thank for the quick reply! I tried to answer your questions as far
as possible (below).
Am 11.06.2026 um 15:45 schrieb Greg Troxel via QGIS-User:
Jochem Kail via QGIS-User <[email protected]> writes:
I would like to reproject a point layer with the CRS being WGS84 (EPSG
4326) to the standard German CRS being ETRS89/UTM32N (EPSG 25832) with
high precision < 1m.
You can't do that, because WGS84 is an ensemble with an intrinsic
accuracy level of 2m. That's the accuracy of the ensemble, before you
consider the accuracy of transformation.
The geodatic datum ETRS89 did only correspond to
WGS84 when it was established in 1989, but given that ETRS89 is
plate-fixed but WGS84 is not, they presently differ by about 1m.
Yes, but surely you are referrring to a particular recent realization of
WGS84 when you say that. Modern WGS84 and modern ITRF are close to the
same thing.
Although some post say that it is possible in QGIS to consider the
epoch (time the data have been mapped) when reprojecting earth-fixed
(non-plate fixed) geographic coordinates to a plate-fixed geodatic
datum, I found no solution to do this in the QGIS GUI.
You could also transform in gdal, if only as a test.
I really would appreciate if someone could explain in detail where and
how to do this transformation in the GUI of QGIS.
That's a fair question and I don't know. Support for epoch is
relatively new in gdal and I am unclear on the state in qgis. You
didn't say what version of qgis you are using, but if it's less than
3.44.11, you are not up to date and it would be good to fix that.
(3.44.x for some x >=4 is probably ok, but hard to tell).
I would ask that you follow up (on list) with:
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).
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.
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".
Why do you believe that your data is accurate to on the order of a
mater?
...because it was taken with a DGPS receiver
How were the coordinates obtained? (As in what kind of measurements,
relative to what sort of controls, and what if any transformations.)
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.
Do the people who managed/prepared this data understand that there are
no methods to obtain accurate coordinates in any realization of 4326
(assuming those people are not in the US Air Force or NIMA, and I'm
not going to help you with military issues!).
No, nor do I ;-).
I realize these questions sound difficult, and I expect you won't get
good answers, but that trying will be helpful.
Greg
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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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