Hi Marco, 

Thanks - now that I know that I need to enter the SRID with the "EPSG:"
prefix, and that I need to use -1 in the Source Datum transformation I
can confirm that it works. 

It would be great, if somewhere in the metadata it would be visible that
this file is now displayed using the nadgrids/GSB file. Otherwise, I
have just to blindly trust, that QGIS does the right thing - which is a
bit scary. 

Do you think it would be possible to add a note in the metadata or
general section of the layer properties confirming that QGIS is using a
datum transformation to display this file? 

I will make sure that the documentation will be improved regarding the
global settings of the datum transformations. 

Thanks,
Andreas 

On 2016-10-04 13:17, Marco Hugentobler wrote:

> 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.hugentob...@sourcepole.ch http://www.sourcepole.ch
> 
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