[PROJ] Switch to proj-datumgrid-geotiff for PROJ 7 ?

[Even Rouault]

> Great, and it would be nice to have a programmatic way to fetch them
> from having run projinfo.

To be pondered. In a Unix style philosophy where each utility has its purpose, 
I wouldn't have imagined projinfo to proceed to download itself.

> What I meant is that in a world where all the grid files are on the
> disk, one can say "show me all the plausible transforms, sorted by some
> error metric".  If the grid file isn't present, I'd still like that
> process to be able to operate the same way, but essentialy throwing an
> execption if a grid file that's needed isn't there -- as opposed to
> pruning pipelines that make sense but which cannot currently be done
> locally. 

What you describe is really how PROJ 6 works:

Extract from output of:
projinfo -s NAD27 -t NAD83 --spatial-test intersects -o PROJ

"""
[snip]
-------------------------------------
Operation No. 4:

DERIVED_FROM(EPSG):1313, NAD27 to NAD83 (4), 1.5 m, Canada - NAD27, at least 
one grid missing

PROJ string:
+proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert 
+xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=ntv2_0.gsb +step 
+proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1

Grid ntv2_0.gsb needed but not found on the system. Can be obtained from the 
proj-datumgrid-north-america package at https://download.osgeo.org/proj/proj-datumgrid-north-america-1.3.zip
"""

> I realize this is a discusion about the pre-RFC4 world, but to
> me this is part of avoiding those not opting into RFC4 becoming
> second-class.

RFC4 is not about making exiting usages second-class. It is about making new 
usages easier/possible.
Requiring grids of "best" transformation to be available when using the black 
box/high level API proj_create_crs_to_crs() (like in cs2cs) is really another 
topic to the one I'd want to address in this thread :-) and a rather 
controversial one. PROJ offers the API for making people aware of what is 
missing and could be used, as used by QGIS 3.10 

Even

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