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


Even Rouault




Implementation target:


Last Updated:



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 is , which has the same content as 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 maintenance 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 As mentioned in , 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 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 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.

Proposed implementation and

Adoption status

The RFC was adopted on 2020-01-28 with +1's from the following PSC members

  • Kristian Evers

  • Even Rouault

  • Thomas Knudsen

  • Howard Butler

  • Kurt Schwehr