Jochem,
Le 24/01/2022 à 20:09, Lesparre, Jochem via PROJ a écrit :
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?
Instead of the target_crs_code metadata item, you could include a
target_crw_wkt with a WKT2 string. That can be done through the
ntv2_to_gtiff.py script as it can accept a WKT2 CRS string as the value
for --target-crs.
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?
You could possibly add with GDAL standard tools (gdal_translate, etc) a
band with the accuracy and a description of "geoid_undulation_accuracy"
(you may need to go through a VRT to manually add the Units to the
band). The validate_vertical_offset_geographic_to_vertical() method of
the check_gtiff_grid.py script would likely have to be updated so that
it doesn't emit an information message about the new band not being
recognized. And the spec at
https://proj.org/specifications/geodetictiffgrids.html as well
* Can I add a recommended_interpolation_method with the Python scripts?
Not currently. Either enhance them or add the metadata item manually
through GDAL tools (gdal_translate -mo
recommended_interpolation_method=foo in.tif out.tif -co COMPRESS=DEFLATE
-co INTERLEAVE=BAND -co PREDICTOR=3). Note that PROJ itself will ignore
that item and always apply bilinear interpolation.
* How should I supply the corrected GeoTIFF files to PROJ?
Pull request against https://github.com/OSGeo/PROJ-data
<https://github.com/OSGeo/PROJ-data/tree/master/nl_nsgi>
* Is it necessary for PROJ to use new files names to distinguish
them from the old version with incorrect metadata?
No
*
Regards, Jochem
[1] https://github.com/OSGeo/PROJ-data/tree/master/nl_nsgi
<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
<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
<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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