[gdal-dev] GDAL 3 and PROJ 6

[Even Rouault]

> Ok, I'll work the proj.db way, it seems to be more profitable to do so. If
> I'm not mistaken, both PROJ 6 and QGIS rely on proj.db to reproject, isn't
> it?

Yes (QGIS 3.8 or later needed to use PROJ 6 new capabilities)

> 
> With PROJ 6 patched with the epsg file, I was trying on a vanilla GDAL
> installation this:
> 
> gdaltransform -s_srs epsg:23030 -t_srs epsg:25830 <<EOF
> 235205.243 4142110.093
> 265467 3988010
> EOF
> 
> Curiously enough, if I use this:
> 
> gdaltransform -s_srs +init=epsg:23030 -t_srs +init=epsg:25830 <<EOF
> 235205.243 4142110.093
> 265467 3988010
> EOF
> 
> it does use the grid and the transformation is decimeter-perfect. However,
> I'm getting a big DEPRECATION warning! Not good.

Yes, init files are now deprecated in PROJ6. And using epsg:xxxx or 
+init=epsg:xxxx is indeed different: the former doesn't try to use a init file 
but a corresponding entry in proj.db, whereas the later will try first to 
locate a epsg init file, and if not found, will fallbak to proj.db.

> So, for not to take much of your time, do you think if I work the original
> patching of PROJ 6 via the proj.db way a vanilla GDAL build will rely on it
> out-of-the-box? Not wanting to abuse, but... any hint of what to expect
> when I come to PostGIS? Will it use PROJ 6 out-of-the-box?

If you use PostGIS 3 beta, it should use proj.db out of the box. Older 
versions will rather use the PROJ strings of the spatial_ref_sys table.

> 
> Besides, if I'm not mistaken, you, Even, are also one of the people behind
> PROJ 6. How difficult can be to contribute the Spanish patch to the source?

Depends exactly on what you intend to do, but
https://github.com/OSGeo/proj-datumgrid/blob/master/CONTRIBUTING.md should 
give you some hints.

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