[gdal-dev] Displaying the transform behind a CoordinateTransformation object

[Daniel Evans]

Hello,

In QGIS, it is possible to select from several available transformations between coordinate systems if the user desires. See their documentation at: https://docs.qgis.org/3.4/en/docs/user_manual/working_with_projections/working_with_projections.html#datum-transformations

With GDAL, is it possible to perform some or all of:

  1.  Showing the details of the transform automatically selected by a CoordinateTransformation object,
  2.  Selecting/specifying an alternate transform,
  3.  Listing all available transforms between two coordinate systems?


I've had a quick look at the QGIS source code, which pulls the transforms from a sqlite database, but the table it references ("tbl_datum_transform") is not in my proj.db (Proj v6.2.1), so I suspect it's a separate/custom/processed database.

https://github.com/qgis/QGIS/blob/f32725658f205af4ac9718d3a3b74f13eaa98d3e/src/core/qgsdatumtransform.cpp
https://github.com/qgis/QGIS/blob/c491b9dbd7dd5012a9db4a9a21ded28ae49bd4ec/src/core/qgscoordinatereferencesystem.h
(A comment in the latter states "The database of CRS shipped with QGIS is stored in a SQLite database (see QgsApplication::srsDatabaseFilePath()) and it is based on the data files maintained by GDAL project (a variety of .csv and .wkt files).")


In particular, my query relates to transforms to/from British National Grid, where I'm told that many transforms of varying accuracy are available, and QGIS doesn't choose the transform our users want automatically. I've not been able to answer their queries regarding what transformation GDAL has chosen when we create a CoordinateTransformation object.

Dr Daniel Evans
Software Developer


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