[PROJ] Some (probably dumb) questions about cs2cs and projinfo

[Andrea Giudiceandrea]

Il 24/02/2026 14:11, Even Rouault ha scritto:
> Why IGM is still not releasing freely their grids ? (I remember doing 
> them some education in Firenze in 2022 ...)

 > This is a case where one could argue which one is the best between the
 > ballpark ED50 -> IGM95 or going through WGS 84 by using a likely poorly
 > conditionned Helmert transformation. Both results are right and wrong to
 > some accuracy requirements...


You are perfectly right. I'm well aware of the issue. IGM sells the 
grids, and, until now, it has not wanted to give up such income making 
them freely available...
Some unofficial grid shift files for Italy are however publicity 
available, which have the same precision of the official ones, but their 
licence and legality is not clear.

Anyway, my concerns and doubts were firstly related to just 
understanding the reason why (although they all use the same PROJ and 
proj.db), cs2cs (and PostGIS and SpatiaLite) uses a particular (better) 
transformation, whereas QGIS another (worst) one, regardless of the fact 
that any of such transformations could be considered the right one.

For that transformation, however, all the 5 available Helmert 
operations, the first one of which is used by cs2cs (and PostGIS and 
SpatiaLite), are much better than the ballpark operation used by QGIS 
(which can only choose between the grid shift operation and the ballpark 
one), since the ballpark is more than 100 metres off.

Regards.

Andrea