[Qgis-user] Struggling with datum transformations and grid shift files

[Marco Hugentobler]

Hi Andi

 >1. As far as I understand, I have to copy the gsb file to the 
directory where proj stores the gsb files, in my case "C:\Program 
Files\OSGeo4W64\share\proj". Alternatively, one could set another folder 
 >and adjust the PROJ_LIB env variable accordingly.

That should work


to 2. and 3. : In the option dialog, you enter the datum transformation 
id of the datum transformation(s), not the proj parameter.
So to get from epsg:21781 to epsg:2056 with CHENYX06a.gsb, you enter:

Source crs: EPSG:21781
Destination crs: EPSG:2056,
Source datum transformation: 100001
Destination datum transformation: -1

An easier way to get to these numbers is to check the box 'Ask for datum 
transformations if no default is defined'. Then add a layer in 21781 to 
the map and transform it to 2056 on-the-fly. In the dialog asking for 
the datum transformation, check 'Remember selection'. The entry will 
then appear in the options dialog.

Regards,
Marco

On 10/04/16 12:58, Neumann, Andreas wrote:
>
> Hi,
>
> I am struggling in QGIS 2x with datum transformations.
>
> The manual is not explicit how to handle this correctly.
>
> Here are the steps I tried:
>
> 1. As far as I understand, I have to copy the gsb file to the 
> directory where proj stores the gsb files, in my case "C:\Program 
> Files\OSGeo4W64\share\proj". Alternatively, one could set another 
> folder and adjust the PROJ_LIB env variable accordingly.
>
> 2. In "Settings" --> "Options" --> "CRS" I can define default datum 
> transformations. I tried to enter 21781 (Source CRS), 2056 
> (Destination CRS), +nadgrids=CHENYX06a.gsb (Source datum 
> transfomation). Destionation datum has been left empty.
>
> 3. I would expect that whenever this combination 21781 --> 2056 shows 
> up that QGIS would ask me to pick one the defaults specified in 2.
>
> The problem is that 2. does not work. When I close the dialogue and 
> reopen it, my modifications are gone ... and I never have the chance 
> to select my datum transformation option.
>
> Can you help me what I am doing things wrong?
>
> Thank you,
>
> Andreas
>
>
>
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-- 
Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch

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