[PROJ] Question about new grid file in Czech Rep.

[Even Rouault]

Javier,

PROJ is not smart enough to use datum transformations involving a 
projected CRS when doing datum transformations between geographic CRS, 
without an explicit record in the database. So here you would need to 
add a concatenated transformation record chaining 
PROJ:ETRS89_3D_TO_S_JTSK_E_N_BALTIC_HEIGHT with the inverse map 
projection of S-JTSK / Krovak East North EPSG:5514

Looking at the cuzk online transformer at 
https://geoportal.cuzk.cz/(S(aipi30p34gvo5u3w1xywxga3))/Default.aspx?lng=EN&mode=TextMeta&text=wcts&menu=19 
, coordinates in S-JTSK can only be given/returned with the projected 
CRS. Perhaps it is not "idiomatic" to use geographic coordinates in that 
system.

Even

Le 06/10/2025 à 21:29, Javier Jimenez Shaw via PROJ a écrit :
> Hi
>
> Playing with the latest master in the Czech Republic, I noticed 
> something strange.
> The new grid file cz_cuzk_table_-y-x_3_v1710.tif is used only if the 
> projected CRS EPSG:5514 is used, but not with its geographic 
> "parent", S-JTSK EPSG:4156. Well, it is at position 13 on the list of 
> operations.
>
> Just compare
> PROJ_NETWORK=ON ./projinfo -o proj --3d EPSG:4258 EPSG:5514+8357 
> --spatial-test intersects
>
> with
>
> PROJ_NETWORK=ON ./projinfo -o proj --3d EPSG:4258 EPSG:4156+8357 
> --spatial-test intersects
>
> Doing only 2D transformation, it is even more clear. The grid file is 
> never used with EPSG:4156
>
> Does it make sense?
>
> I know that the grid file is defined in EPSG:5514. But I assume that 
> the correction applies to the geographic system transformation, not to 
> the Krovak projection itself.
>
> Cheers
> Javier.
>
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