[PROJ] Time dependency in ETRFxx to ETRFyy coordinate transformations?

[Even Rouault]

Hi,

I was reading 
https://lists.osgeo.org/pipermail/qgis-user/2026-June/056346.html where 
Greg mentions "Also use command-line proj:  $ projinfo -s ETRF2000 -t 
ETRF2020 which returns a lot of operations that surprise me, being 
time-dependent transforms with non-zero rates".

Indeed PROJ does that transformation using a ITRF2020 pivot, which is in 
line with the EUREF technical note 1 
(http://etrs89.ensg.ign.fr/pub/EUREF-TN-1-Mar-04-2024.pdf). But the 
consequence of using such time-dependent transforms is that the results 
depends *and change* on a coordinate epoch.  So ETRFxx frames are not 
"static" ? From a number of things seen in past years, it looks to me 
that this opposition of static vs dynamic frames is still very fuzzy and 
perhaps is not relevant at all. On the US side their recent renaming of 
NATRF2022 to NATRF2022e2020 (epoch 2020.0) also shows similar confusion.

$ echo 49 2 0 2000 | bin/cs2cs -d 8 ETRF2000 ETRF2020
49.00000061    1.99999942 0.00346292 2000
$ echo 49 2 0 2015 | bin/cs2cs -d 8 ETRF2000 ETRF2020
49.00000065    1.99999955 0.01117636 2015
$ echo 49 2 0 2020 | bin/cs2cs -d 8 ETRF2000 ETRF2020
49.00000067    1.99999960 0.01374751 2020

The good news is that while playing with the online  ETRF/ITRF 
Coordinate Transformation Tool (ECTT)  tool 
(https://epncb.oma.be/_productsservices/coord_trans/index.php#results), 
both this tool and PROJ agree at the tenth of millimeter on a few random 
points I tried, and when checking the " show intermediate steps" tick 
box, one can also see they use a ITRF2020 intermediate (actually they do 
ETRF2000 -> ITRF2000 -> ITRF2020 -> ETRF2020 at the specified epoch.  
But the ETRF2000 -> ITRF2020 direct transformation we have in PROJ must 
be the compaction of ITRF2000 -> ITRF2000 -> ITRF2020)

/me still not having any clue at geodesy ;-)

Even

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