[Qgis-user] Reproject from WGS84 (EPSG: 4326) to ETRS89/UTM32N (EPSG: 25832) considering the epoch

[Jochem Kail]

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 <qgis-user at lists.osgeo.org> 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

jochem.kail at uni-due.de
https://www.uni-due.de/aquatische_oekologie/mitarbeiter/jochem_kail.php

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