[Qgis-user] NTv2-transformation: different output values in QGIS and GDAL

[Even Rouault]

Le 16/01/2024 à 11:59, andy via QGIS-User a écrit :
> Hi,
> I'm doing NTv2-transformation, using this gsb file:
> https://web.archive.org/web/20201018143532/https://provincia.agrigento.sitr.it/scaricabili/NTv2/ItalyED50toWGS84_NTV2_GN.gsb
>
> I have copied it in "C:\OSGeo4W\share\proj" and I have defined this 
> custom projection:
> +proj=utm +zone=33 +ellps=intl +nadgrids=ItalyED50toWGS84_NTV2_GN.gsb 
> +units=m +no_defs +type=crs
>
> If I test it in QGIS, in the custom CRS window, using lon=13 and 
> lat=38, I have in output:
> 324388.7743
> 4207772.1972

Did you try with the full path 
+nadgrids=C:\OSGeo4W\share\proj\ItalyED50toWGS84_NTV2_GN.gsb ?

If I do that (well on Linux: "+proj=utm +zone=33 +ellps=intl 
+nadgrids=/home/even/qgis/qgis/build_ubuntu2204/ItalyED50toWGS84_NTV2_GN.gsb 
+units=m +no_defs"), I do get the correct result 324454.6136 
4207894.9261 in QGIS (master)

But, what is the history of this "ghost" grid that requires fetching it 
from web.archive.org ? Has it had some official status, and could it be 
provided under an open data license ? If so, it would be great if 
Italian authorities could register to IOGP/EPSG this grid for ED50 -> 
WGS 84 for Italy... Then we could ship it in PROJ-data.

But I remember complicated discussion at FOSS4G Firenze with the guys 
from the geodetic authority (https://www.igmi.org).... Not sure if that 
has lead to anywhere.

Even

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