[PROJ] Correcting metadata in GeoTIFF files

[Lesparre, Jochem]

Dear list,



I'm from Netherlands Partnership Geodetic Infrastructure (NSGI). We published NTv2 (.gsb) and VDatum (.gtx) grid files for the national CRS of the Netherlands (RD coordinates with NAP height) in 2019. PROJ converted these to GeoTIFF (.tif) in 2020 [1]. Unfortunately, some wrong information was stored in the metadata of the GeoTIFF files in the conversion. We are now creating new GeoTIFF files with corrected metadata, using the Python scrips ntv2_to_gtiff.py [2] and vertoffset_grid_to_gtiff.py [3].



For most changes in the metadata this is straightforward, but there is one more difficult issue:

We have two variants of the horizontal transformation:

  *   Conventional 1-step transformation (variant 2): geographic coordinates of national CRS ---[nl_nsgi_rdtrans2018.tif]---> ETRS89
  *   Better 2-step transformation (variant 1): geographic coordinates of national CRS ---[nl_nsgi_rdcorr2018.tif]---> corrected geographic coordinates of national CRS ---[7_parameter_transformation]---> ETRS89

Since there is no separate EPSG code for the corrected geographic coordinates of the national CRS, we want to use the same EPSG code for both corrected and uncorrected coordinates. Or will it give problems when the source and target CRS of a GeoTIFF file are the same?



Next to this, I have some other questions:

  *   Shouldn't a GeoTIFF grid file for a vertical transformation have an accuracy band like a grid for a horizontal transformation? Is it possible with the Python script to create an accuracy band in the GeoTIFF from a VDatum grid file?
  *   Can I add a recommended_interpolation_method with the Python scripts?
  *   How should I supply the corrected GeoTIFF files to PROJ?
  *   Is it necessary for PROJ to use new files names to distinguish them from the old version with incorrect metadata?



Regards, Jochem



[1] https://github.com/OSGeo/PROJ-data/tree/master/nl_nsgi

[2] https://github.com/OSGeo/PROJ-data/blob/master/grid_tools/ntv2_to_gtiff.py

[3] https://github.com/OSGeo/PROJ-data/blob/master/grid_tools/vertoffset_grid_to_gtiff.py





J. Lesparre

Netherlands Partnership Geodetic Infrastructure (NSGI.nl)






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