[PROJ] EPSG:4326 --> EPSG:5677

[Even Rouault]

Roger, (sorry for the first name error in my previous email)


>
>
> I see that three possible operations are listed. To be honest, I do 
> not know which one is most appropriate.
No worry. You're not alone. It depends on the context...
> I am guessing that #1 is the default.
The grid based has the best accuracy, so when the grid is available, it 
is indeed used
> How would I choose one of the other sets of operations? Use the listed 
> strings as the destination CRS? Or is there some other way to select 
> it? Should the reported altitude always be over the new ellipsoid?

If you want 3D operations, you should use the 3D versions of the CRS. 
You can add the --3d flag to projinfo to force promotion. But in the 
case of DHDN there's no official 3D version of the CRS in the EPSG 
dataset, so what PROJ will report may be dubious.

If you try "projinfo EPSG:4326 EPSG:5677 --spatial-test intersects 
--grid-check none  --3d --single-line -o PROJ", the second operation 
will be:

+proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert 
+xy_in=deg +z_in=m +xy_out=rad +z_out=m +step +proj=cart +ellps=WGS84 
+step +inv +proj=helmert +x=598.1 +y=73.7 +z=418.2 +rx=0.202 +ry=0.045 
+rz=-2.455 +s=6.7 +convention=position_vector +step +inv +proj=cart 
+ellps=bessel +step +proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 
+y_0=0 +ellps=bessel

And you can create a PJ object from it with

myTransf = proj_create(ctxt, "+proj=pipeline +step +proj=axisswap 
+order=2,1 +step +proj=unitconvert +xy_in=deg +z_in=m +xy_out=rad 
+z_out=m +step +proj=cart +ellps=WGS84 +step +inv +proj=helmert +x=598.1 
+y=73.7 +z=418.2 +rx=0.202 +ry=0.045 +rz=-2.455 +s=6.7 
+convention=position_vector +step +inv +proj=cart +ellps=bessel +step 
+proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel")

>
>
>
>     Even
>
>     Le 24/09/2025 à 08:57, Roger Oberholtzer via PROJ a écrit :
>>
>>
>>     I am trying to project some EPSG:4326 locations (from a GPS) to
>>     EPSG:5677 (DHDN Germany Zone 3). I have sample data from the
>>     German road authorities (BASt). They feel that the locations
>>     provided by proj (9.6.0) are consistently 1.5 meters or so away
>>     from that they expect. They were also expecting a change in
>>     altitude as the ellipsoid was different. I have implemented this
>>     with something like this:
>>
>>     myCRS = proj_create_crs_to_crs( 0,  "EPSG:4326", "EPSG:5677",  0);
>>
>>     And then calling proj_trans(myCRS, PJ_FWD, PJ_COORD);
>>
>>     My question is if I need to do more than this as EPSG:5677 is
>>     ellipsoid Bessel 1841.
>>     -- 
>>     Roger Oberholtzer
>>
>>     _______________________________________________
>>     PROJ mailing list
>>     PROJ at lists.osgeo.org
>>     https://lists.osgeo.org/mailman/listinfo/proj
>
>     -- 
>     http://www.spatialys.com
>     My software is free, but my time generally not.
>
>
>
> -- 
> Roger Oberholtzer

-- 
http://www.spatialys.com
My software is free, but my time generally not.
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