On 2016-10-24 06:23, Neumann, Andreas wrote:

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:
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?


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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