[QGIS-Developer] Dealing with datum shift grids in QGIS

Kristian Evers kreve at sdfe.dk
Thu Oct 11 11:31:13 PDT 2018

On 11 Oct 2018, at 18:55, Martin Dobias <wonder.sk at gmail.com<mailto:wonder.sk at gmail.com>> wrote:

Regarding contents of proj.db table, will it be acceptable for apps
like QGIS to browse its contents or should it be considered as Proj
library's implementation detail and any queries to it should go
through Proj API? I am thinking it would be nice to automatically
determine grid packages for missing grid files and to download them in
QGIS if the users wants. Another question is what to do with grids
with license that does not allow redistribution - should we have
entries with URL in grid_transformation table for such grid files as
well? (maybe with a flag that it is not direct link to a package, just
a web page about that grid)

Some time ago Even and I briefly discussed the possibility of having
PROJ offer a direct link to grids that are needed for a transformation
but not available on the system. This should let apps like QGIS deal
with grids quite smoothly I think.

This would compliment or perhaps even replace the current
proj-datumgrid packages that we maintain at the moment. It would
still be based on a curated set of grids making sure that they are
free to distribute. For the ones that are not available for redistribution,
well… I guess the makers of those grids don’t really want you to use
them in the first place :-) In all seriousness though, there’s not much
we can do. They are quite often behind a login so direct links can’t
be offered. We do keep a non-exhaustive list of non-free grids here:


If a feature like this  is something that might be useful to users of
PROJ I we should look into this more closely.

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