[Qgis-user] QGIS server: configuration of grid shift files for datum conversions

Fri Oct 28 07:50:15 PDT 2016

Hi again, Andy, 

Just trying to understand where the .gsb files are actually referenced.
The table tbl_datum_transformation does not contain any references to
the .gsb files. How does QGIS find those files? 

Thanks for any hints, 

Andreas 

On 2016-10-24 11:09, Schmid Andreas wrote:

> Hi,
> 
> At Kanton Solothurn we had the same requirement. And we did not want to fiddle with the original proj.4 definition files. So we paid Sourcepole for solving this directly in QGIS Server (https://github.com/qgis/QGIS/commit/70863ecaf0ccfcb538e3892af4b528304b21a0a2).
> This is how it works under Ubuntu:
> 
> * Place those CHENYX06a.gsb and CHENYX06_etrs.gsb in /usr/share/proj (probably in our case only CHENYX06a.gsb is actually needed)
> 
> * If QGIS Server was installed before placing the .gsb files, run /usr/lib/qgis/crssync (or run it anyway)
> 
> * In /etc/apache2/mods-enabled/fcgid.conf place the following additional option:
> InitialEnv DEFAULT_DATUM_TRANSFORM=EPSG:21781/EPSG:2056/100001/-1;EPSG:2056/EPSG:21781/-1/100001
> 
> The meaning of this option is: when your .qgs is in EPSG:21781 and the WMS request is in EPSG:2056, then use datum transformation 100001; when your .qgs is in EPSG:2056 and the WMS request is in EPSG:21781, then use datum transformation 100001 as well, but use it the other way round.
> The datum transformation code 100001 can be looked up in /usr/share/qgis/resources/srs.db. Look at attribute coord_op_code of table tbl_datum_transform.
> 
> Regards,
> Andy
> 
> -----Ursprüngliche Nachricht-----
> Von: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] Im Auftrag von Richard
> Duivenvoorde
> Gesendet: Montag, 24. Oktober 2016 10:01
> An: Neumann, Andreas
> Cc: QGIS User List
> Betreff: Re: [Qgis-user] QGIS server: configuration of grid shift files for datum
> conversions
> 
> On 2016-10-24 06:23, Neumann, Andreas wrote: Hi,
> 
> I have to configure QGIS server to properly handle grid shift files.
> In Switzerland we need to offer "old" and "new" coordinates (different
> datums) at the same time with our services and the conversion needs to
> be done with grid shift files or tin based interpolation.
> 
> I know that I have to copy the CHENYX06a.gsb file to my proj lib, e.g.
> /usr/local/shar/proj  - but how can I make QGIS Server to actually use
> this file? I assume that I need to copy an additional config file or
> SRS/grid shift file combinations to QGIS server and/or specify an
> environment variable - right?
> 
> Are there any resources explaining how I can configure QGIS server to
> make use of these grid shift files?
> 
> Thank you for any pointers! 
> Hi Andreas,
> 
> funny, in NL we just had a discussion about how to handle different
> available datum transformation too last week...
> 
> we also recently got a grid shift file to better handle datum
> transformation and I found this post of Sourcepole:
> http://blog.sourcepole.ch/2014/02/18/ntv2-transformations-with-qgis/
> Which I think answers your question (at least for the desktop side)
> 
> But from a mini presentation from somebody from our National Cadastre, I
> actually draw the conclusion that the way QGIS represents CRS's is too
> narrow/not complete...
> 
> Please correct me if I'm wrong, but a lot of 'On The Fly'-reprojections
> actually need 2(!) proj configurations/steps:
> - the projection parametes (mostly available in the epsg database)
> - the datum transformation parameters (these is the 'towsg84...'-part,
> AND only needed when you do an actual datum transformation, PARTLY
> available in the epsg database)
> The last part can either be a list of parameters (to calculate it) OR
> (as in your case and in the dutch preferred case) a grid shift file (as
> proj can only handle one of these).
> 
> SO: for example for certain crs's (in QGIS the dutch one is called
> epsg:28992) there is actually a SET of combinations available: the
> projection and 4 or 5 transformations
> (http://pix.toile-libre.org/upload/original/1477238061.png),
> 
> QGIS actually picks one (from qgis.db) of these and present them to the
> user as THE epsg:28992 crs, while in our case this is actually not the
> best fit, and in your case you actually want to pick two!
> 
> I understood that in ArcGIS, the user actually every time is presented
> with a 'pick the right datum transformation' dialog as soon as the data
> needs a datum transformatio. As a draw back: making it more difficult
> for new users to pick the right one, BUT on the positive side making it
> easier to pick the right projection/datumtransformation pair!
> 
> So what about using both worlds:
> - in the qgis.db / crs table NOT use the projection-name as
> crs-identifier (so NOT epsg:28992), BUT the combination of projection
> AND datumtransformation, so in our case (28992_4833, 28992_15934, or
> epsg28992_epsg4833, epsg28992_qgis0001 to be able to handle
> projection/transformation combinations not available in the epsg db)
> - and instead of current possibility to have a 'check'-box to have the
> 'select datum transformation' popup we make this available in the
> 'choose crs'-dialog with a button, and then 'remember' the last used
> crs's (in which a crs is NOT only the projection anymore but a
> projection/transformation combi)
> 
> IF QGIS starts using this combinations AND proj/gdal will ship all
> gridfiles towgs-paramters etc etc, this will become a better world :-)
> 
> Does this make sense? Do others get excited from this idea too?
> 
> Regards,
> 
> Richard Duivenvoorde
> 
> ps about providing the gridfiles, I urge our national Kadaster to make
> it available as treu open geodata, that is without a copyright
> disclaimer, so it can be picked up by Debian and other distro's.
> 
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