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In QGIS there is an entry in the tbl_datum_transform for a NTv2-grid called BWTA2017.gsb ( https://github.com/qgis/QGIS/pull/6980 ).

So theoretically it would be enough just to activate the option "Ask for datum transformation when no default is defined" and choose the grid the first time a transformation is done.

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I wonder which parts of QGIS will still not make use of the chosen datum transformation that's stored in the QGIS-settings (batch-transformation? raster-transformation?).

For which use-cases is it neccessary to also update the table-entry of tbl_srs like this:

update tbl_srs set parameters='+proj=tmerc +lat_0=0 +lon_0=9 +k=1 +x_0=3500000 +y_0=0 +ellps=bessel +datum=potsdam +nadgrids=@BWTA2017.gsb,@BETA2007.gsb,null +units=m +no_defs' where srid=31467

I think the documentation of QGIS is lacking information about a crucial part of working with it.

An already asked (less specific) question concerning some generell informations about integrating NTv2-Grids in QGIS has not led to an answer from credible and/or official sources yet (even if there was a bounty offered):

Integrating NTv2-transformation in QGIS?

  • I think the kicker is "when no default is defined". You would need to identify all existing "local" GCS/PCS and make sure the definitions don't have +towgs84 option and/or change them to use the ntv2 file. – mkennedy Nov 7 '18 at 19:47

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