[gdal-dev] GDAL 3 and PROJ 6

[Even Rouault]

Juan Pedro,

> I maintain a set of Docker containers of PostGIS and currently I'm working
> on a new release with updated version of the stack, which include PROJ 6,
> GDAL 3, PostgreSQL 11.5, and PostGIS 2.5.3. I know PROJ 6 is a game
> changer, and all the work I've done in the past configuring PROJ 4 to my
> country (Spain) transformations with GSB grids might have been deprecated.
> It appears so! Hopefully, after attempting a lot of things, I was able to
> make PROJ 6 work nicely with the GSB by providing a resource epsg file.

Creating a 'epsg' text resource file is now deprecated, in favor of using the 
'proj.db' database. See below
 
> My hopes were that, once PROJ 6 was in place, GDAL 3 would make use of it
> for gdaltransform, but somehow GDAL is not playing nicely with PROJ 6 and
> transformations are not using the grids. If someone can shed some light
> regarding this issues, I'll be most grateful:
> 
> - what is the relation between GDAL 3 and PROJ 6? I mean, if cs2cs is using
> the grid to transform, shouldn't GDAL rely on PROJ for reprojection and do
> it well too?

Yes, normally if something works with cs2cs it should work with GDAL 
reprojection API too, but they don't exactly use the same PROJ API, so details 
may matter. Can you give a full reproducer of your issue ?

> - if GDAL has to be configured independently, what's the proper way to
> attach a grid to a EPSG code so reprojections using the epsg:XXXXX syntax
> would work with the grid?

This is doable by addinng a new entry in the 'grid_transformation' table of 
the proj.db file.

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