[PROJ] Datum Grids, Which Ones?

[Kristian Evers]

Yes, this could do with some clarification in the docs. Briefly, it goes something like this:

proj-datumgrid is the original datumgrid packages. It contains mostly basic North American grids,
a few European grids and the egm96 geoid model. The test suite will fail without this. Many
transformations will work without it. These days this package is mostly kept for legacy reasons.

The regional grid packages contain grids and other resources that are useful in those regions.
Currently three regions are covered. The grids are split in these packages to avoid consuming
too much disk space for users who has that concern. If you have plenty of space just get all of them.

The world package is for grids that has global coverage. Currently only the egm08_25.gtx grid is
in that package. It is NOT a package containing all grids of the world.


We have previously talked about adding a -all package which holds everything. This will be easier
for most people to use it seems. I haven’t gotten around to it because it requires changing the
CMake setup of the proj-datumgrids repository [0] and that is just not a job that I am particularly keen
on doing. A pull requests adding this is welcome.

Finally, Howard is trying to find the funds to deal with grids in a smarter way. I suggest that those
who can support his effort.

/Kristian

[0] https://github.com/OSGeo/proj-datumgrid


> On 30 Sep 2019, at 22:09, Paul Ramsey <pramsey at cleverelephant.ca> wrote:
> 
> The list of datum grids on the web side is not 100% clear on what grids a person should pull for what purpose…
> There’s regional files (europe and na and oceania) there’s a “world” file which, by size, perhaps includes all of the above? and there’s the un-specialized file, proj-datumgrid that is… everything not in the other files? something else?
> 
> 
> • 2018-09-15 proj-datumgrid-1.8.zip
> 
> • 2019-09-01 proj-datumgrid-europe-1.4.zip
> 
> • 2019-03-01 proj-datumgrid-north-america-1.2.zip
> 
> • 2018-03-01 proj-datumgrid-oceania-1.0.zip
> 
> • 2019-03-01 proj-datumgrid-world-1.0.zip
> 
> If the best practice is just “download’em’all if you have the space”, then an “all” might be good, instead of “world”. And some note on what proj-datumgrid-1.8.zip actually is? Is it for Proj < 6 only?
> 
> P
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