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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Hello,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">In QGIS, it is possible to select from several available transformations between coordinate systems if the user desires. See their documentation at:
<a href="https://docs.qgis.org/3.4/en/docs/user_manual/working_with_projections/working_with_projections.html#datum-transformations">
https://docs.qgis.org/3.4/en/docs/user_manual/working_with_projections/working_with_projections.html#datum-transformations</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">With GDAL, is it possible to perform some or all of:<o:p></o:p></span></p>
<ol style="margin-top:0cm" start="1" type="a">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-family:"Arial",sans-serif">Showing the details of the transform automatically selected by a CoordinateTransformation object,<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-family:"Arial",sans-serif">Selecting/specifying an alternate transform,<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-family:"Arial",sans-serif">Listing all available transforms between two coordinate systems?<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><a href="https://github.com/qgis/QGIS/blob/f32725658f205af4ac9718d3a3b74f13eaa98d3e/src/core/qgsdatumtransform.cpp">https://github.com/qgis/QGIS/blob/f32725658f205af4ac9718d3a3b74f13eaa98d3e/src/core/qgsdatumtransform.cpp</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><a href="https://github.com/qgis/QGIS/blob/c491b9dbd7dd5012a9db4a9a21ded28ae49bd4ec/src/core/qgscoordinatereferencesystem.h">https://github.com/qgis/QGIS/blob/c491b9dbd7dd5012a9db4a9a21ded28ae49bd4ec/src/core/qgscoordinatereferencesystem.h</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">(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).”)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-family:"Arial",sans-serif;color:black;mso-fareast-language:EN-GB">Dr Daniel Evans<o:p></o:p></span></b></p>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#F6A124;mso-fareast-language:EN-GB">Software Developer<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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