[PROJ] Could you please explain me why I can't project RDN2008 (EPSG:6706) to ERTF2000 (EPSG:9067) or ITRF2000 (EPGS:8997)

[Even Rouault]

Giacomo,

Coordinate transformations are definitely not a trivial subject, so no 
worry not understanding what's going on here.

PROJ uses the content of its database to find, or infer, coordinate 
transformations. Most of the content comes from the EPSG database itself.

For RDN2008, there are 2 transformations available:

- one from RDN2008 to ETRS89 (EPSG:4258), with a null Helmert 
transformation and a accuracy of 0 m. This is consistant with how you 
described RDN2008, and this transformation has the comment "RDN2008 is 
the second Italian realization of ETRS89"

- one from RDN2008 to WGS 84, which is the same null Helmert 
transformation, copied from the above, but with a accuracy of 1 m (due 
to WGS 84 being dynamic and moving over time compared to plate fixed CRS 
such as RDN2008 or ETRS89)

The first one would be a good candidate, but PROJ doesn't know he can 
use it, since there is no explicit transformation in its database 
between ETRS89 and ETRF2000. That said in the database, ETRS89 is 
modeled as a datum ensemble of the many ETRFxxxx realizations, so 
potentially PROJ could use that information to infer a null 
transformation between ETRS89 and ETRF2000 with the accuracy of the 
datum ensemble (here 0.1 m) (we actually do that manually for the WGS 84 
datum ensemble vs its realizations, but that should be generalized), and 
chain that together with the RDN2008 to ETRS89 transformation.

In the meantime, you can customize your proj.db by adding the following 
2 records. Can be done with the sqlite3 command line utility

$ sqlite3 proj.db

INSERT INTO "helmert_transformation" 
VALUES('PROJ','ETRS89_TO_ETRF2000','ETRS89 to ETRF2000','Accuracy 0.1m, 
derived from ETRS89 datum ensemble definition','EPSG','9603','Geocentric 
translations (geog2D 
domain)','EPSG','4258','EPSG','9067',0.1,0,0,0,'EPSG','9001',NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,'',0);
INSERT INTO "usage" VALUES(
     'PROJ',
     'ETRS89_TO_ETRF2000_USAGE',
     'helmert_transformation',
     'PROJ',
     'ETRS89_TO_ETRF2000',
     'EPSG','1298', -- Europe
     'EPSG','1024'  -- unknown
);

.quit

With that, you'll get

$ projinfo -s RDN2008 -t ETRF2000
Candidate operations found: 1
-------------------------------------
Operation No. 1:

unknown id, RDN2008 to ETRS89 (1) + ETRS89 to ETRF2000, 0.1 m, Italy - 
onshore and offshore; San Marino, Vatican City State.

PROJ string:
+proj=noop

Even

Le 19/07/2022 à 03:02, Giacomo Cappellini a écrit :
> Hello,
>
> I'm generally new to working with CRS different from EPSG:4326 + UTM 
> and I'm facing some issues while trying to use RDN2008, which is the 
> official reference system for Italy at current date.
> Please excuse me if the question sounds trivial to experts, but I 
> can't figure out why there are no non-ballpark operations to go 
> from/to RDN2008 and ETRF2000 or ITRF2000.
>
> Afaik, RDN2008 is ETRS89 realization ETRF2000 epoch 2008.0.
>
> Proj Rel. 8.2.1, January 1st, 2022
>
>     $ projinfo -s EPSG:6706 -t EPSG:9067 --spatial-test intersects
>     --grid-check none --hide-ballpark --summary
>     Candidate operations found: 0
>
>
>     $ projinfo -s EPSG:6706 -t EPSG:8997 --spatial-test intersects
>     --grid-check none --hide-ballpark --summary
>     Candidate operations found: 0
>
>
> Thanks
>
> G.C.
>
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
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