# PROJ RFC 5: Adopt GeoTIFF-based grids for grids delivered with PROJ¶

Author

Even Rouault

Contact

even.rouault@spatialys.com

Status

Implementation target

PROJ 7

Last Updated

2020-01-28

## Motivation¶

This RFC is a continuation of PROJ RFC 4: Remote access to grids and GeoTIFF grids. With RFC4, PROJ can, upon request of the user, download grids from a CDN in a progressive way. There is also API, such as proj_download_file() to be able to download a GeoTIFF grid in the user writable directory. The content of the CDN at https://cdn.proj.org is https://github.com/OSGeo/PROJ-data , which has the same content as https://github.com/OSGeo/proj-datumgrid converted in GeoTIFF files. In the current state, we could have a somewhat inconsistency between users relying on the proj-datumgrid, proj-datumgrid-[world,northamerica,oceania,europe] packages of mostly NTv2 and GTX files, and what is shipped through the CDN. Maintaining two repositories is also a maintaince burden in the long term.

It is thus desirable to have a single source of truth, and we propose it to be based on the GeoTIFF grids.

## Backward compatibility¶

This change is considered to be mostly backward compatible. There might be impacts for software using proj_coordoperation_get_grid_used() and assuming that the url returned is one of the proj-datumgrid-xxx files at https://download.osgeo.org. As mentioned in https://lists.osgeo.org/pipermail/proj/2020-January/009274.html , this assumption was not completely bullet-proof either. There will be impacts on software checking the value of PROJ pipeline strings resulting proj_create_crs_to_crs(). The new grid names will now be returned (the most impacted software will likely be PROJ’s own test suite)

Although discouraged, people not using the new proj-datumgrid-geotiff-XXX.zip archives, should still be able to use the old archives made of NTv2/GTX files, at least as long as the PROJ database does not only point to a GeoTIFF grid. So this might be a short-term partly working solution, but at time goes, it will become increasingly non-working. The nominal combination will be PROJ 7.0 + proj-datumgrid-geotiff-1.0.zip

## Testing¶

PROJ test suite will have to be adapted for the new TIFF based filenames.

Mechanism to auto-promote existing NTv2/GTX names to TIFF ones will be exercised.