[PROJ] Management of datum grid files.

[Chris Crook]

Hi All



I am looking at installing 16 grids for New Zealand vertical datums into proj-datumgrid, and it has raised some questions on the structure and maintenance of this in the future.



Given the current level of interest in grids, and the increasing importance and acquistion of geoid grids, plus (hopefully) extension to include deformation models comprising multiple grids, I am thinking that the number of grids in use may start to grow rapidly.



Following the current convention I would install my grids into proj-datumgrid-oceania, which currently holds a number of grids of relating to Australian datums.  I doubt that many Australian users will be interested in the NZ grids, and vice versa.  (Particularly of course as we do not share a land boundary).



Also for the most part (as far as  I can see) there is not particular packaging around the datumgrid products (other than conda).  So for the most part users will need to download the zip file from the URL on the PROJ pages, and install manually.



So I am wondering if this is a sensible approach.



Initially I thought I wondered about create a directory in proj-datumgrid for NZ grids and a  new product proj-datumgrid-nz.   But since LINZ (NZ geodetic agency) will be publishing these in any case, would it make more sense for LINZ to maintain and host the proj-datumgrid-nz product, and for that simply to be referenced in proj documentation?



The current initiative to deliver grids via a CDN (https://github.com/rouault/PROJ/blob/rfc4_remote_and_geotiff_grid/docs/source/community/rfc/rfc-4.rst) will simplify this, but  the current proposal doesn't replace proj-datumgrid so I think this will be an issue for a bit longer.



To make this work well it would be useful to have a command line tool for installing grids that is installed with PROJ.  Say (sudo) install_proj_datumgrid <region>.   This could have a configuration file which defines the URL of the authoritative source for each region so that "install_proj_datumgrid nz" would download and install a file from (say) www.geodesy.linz.govt.nz/download/proj-datumgrid/proj-datumgrid-nz.zip<http://www.geodesy.linz.govt.nz/download/proj-datumgrid/proj-datumgrid-nz.zip>.  This could work well with the existing products too, eg install_proj_datumgrid oceania.   (This could be a trivial shell script for linux - sure for other OS targets)



With this approach the only thing LINZ would need to maintain within the PROJ infrastructure would be the url of the proj-datumgrid-nz zip file.



Comments?



Cheers

Chris Crook

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