[PROJ] some questions about the Barn Raising

[Even Rouault]

> Thank you for seeing this "problem" with the new CR-SIRGAS system (8908).
> but as I see the problem was a bad entry in the EPSG database. If it is not
> a lot of daring, can I ask you how the IOGP confirmed this problem?

I emailed m contact at IOGP who confirmed my analysis and said they will issue 
a v9.6.2 release with the fix soon. Anyway PROJ is also now able to deal with 
this slightly odd configuration too.

> however, I have a doubt, according to what I have read, "PROJ: SQLite-based
> version of the EPSG database" is the one that maintains the database of the
> EPSG that is used in GDAL.

GDAL 2.5 and libgeotiff 1.5 no longer have their own CRS database as they used 
to have before, and will rely on the database provided with PROJ.

> 
> How does this work in end-user software such as PostGIS and QGIS?
> this database is replicated in these softwares until the implementation of a
> new PROJ say PROJ6? or get a pointrelease CRS database update?

That's a good question. From what I've understood, PostGIS will still ship its 
spatial_ref_sys.sql database (since it is part of its user visible surface), 
and will so have to refresh it regularly from the PROJ database.
For QGIS, I think the intent will be to use ultimately the PROJ database, but 
as QGIS allows customizations, I'm not sure how/when this will be done. PROJ 6 
has some mechanism to attach a supplemental database to the official one. I 
did that with QGIS use case in mind, but this hasn't be field tested yet, so 
might require some additional dev.

> 
> What I want to ask is: how much time or in what cycles of development in
> softwares like QGIS i.e. the databases of the CRS are updated?

Will require all QGIS builds to use PROJ 6, so likely not immediately, and as 
said above, will also require they no longer ship their own QGIS db. I can't 
give a timeframe. Depends on too many external factors.

> 
> I know that in this commit https://github.com/OSGeo/proj.4/pull/1368 (25 mar
> 2019)
> the PROJ DB was updated with the new base of the EPSG.
> 
> from here how do other softwares incorporate this update? a new version of
> PROJ should be adopted as PROJ6 to include this new database. for example,
> you should wait until QGIS 3.8 or PostGIS 3.0 to see the new CR-SIRGAS
> system (8908). 

To have it officially, yes (and provided that the QGIS build do use PROJ 6. 
QGIS 3.8 will stil be able to use older PROJ versions). Otherwise, if you want 
to have it with previous versions of the software, you could add manually a 
new entry in the respective "databases" of the existing softwares. And you 
might also possibly abuse the +towgs84 mechanism to have the CR05->CR-SIRGAS 
datum shift.

> What happens with LTR versions like QGIS 3.4? Do they receive
> the update of the database? 

That would require using the old workflow to refresh the CRS database(s). 
Could technically be done (eh, this is FOSS, so almost anything can be done 
!), but I doubt anyone from the core dev teams will spontaneously invest time 
in doing that.

Even

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