[Qgis-developer] How do datum transformations work?

[Neumann, Andreas]

Hi, 

I am struggling to find out how datum transformations work -
specifically also under QGIS Server. 

I have a .gsb file (CHENYX06a.gsb) for Swiss datum transformations. It
is located under /usr/local/shar/proj/ where all the other .gsb files
are. 

I also set the DEFAULT_DATUM_TRANSFORM variable variable in my
fcgid.conf file: 

  FcgidInitialEnv DEFAULT_DATUM_TRANSFORM
"EPSG:21781/EPSG:2056/100001/-1;EPSG:2056/EPSG:21781/-1/100001" 

I checked the entries in the srs.db on the server in the table
tbl_datum_transform which has an entry: 

wkt_geom coord_op_code epsg_nr source_crs_code target_crs_code
coord_op_method_code p1 p2 p3 p4 p5 p6 p7 remarks scope preferred
deprecated area_of_use_code
NULL 100001 15486 4149 4150 9615 0 

But how does QGIS know that it should use the CHENYX06a.gsb file as
grid-shift file? 

Where exactly does it find this information? 

Thank you for shedding some light on this mystery. 

Andreas 

  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-developer/attachments/20170210/d5937819/attachment.html>