[PROJ] Correcting metadata in GeoTIFF files

[Lesparre, Jochem]

Hi Even and Bas,

The PROJ <6 behaviour for +nadgrids and +geoidgrids in one PROJ4-style string was confusing: Without +nadgrids it required a slightly different geoid grid (e.g. the difference between the nlgeo2008/2018 and naptrans2008/2018 VDatum grids for the Netherlands). It became even more confusing by the “fix” of the behaviour in PROJ >=6. However, this is only a problem for legacy users. For users of PROJ >=6 the behaviour for PROJ4-style strings is better: The same grid file can be used for +geoidgrids with or without +nadgrids. So, I think it is better to leave the behaviour like it is.

Since GeoTIFF grids are not supported in PROJ <6 and the naptrans2008/2018 grids of the Netherlands are only needed with the PROJ <6 behaviour, there is no need for naptrans2008.tif or naptrans2018.tif. Could I ask to remove nl_nsgi_naptrans2018.tif (and naptrans2018.tif, if present) from PROJ (and keep the .gtx versions, if present in PROJ)? Can I do this with the same pull request as replacing some other GeoTIFF grid files for the Netherlands to correct metadata?

I think it would be a good idea to change the way a interpolation CRS should be specified in a Geodetic TIFF Grid. The only mentioned example of this was naptrans2008 and this grid is no longer needed, so this change seems not very urgent if naptrans2008.tif and naptrans2018.tif grids are removed. However, we should at least edit the documentation so it no longer mentions naptrans2008 as example in: https://proj.org/specifications/geodetictiffgrids.html

Regards, Jochem


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