[Qgis-developer] Datum transformations

[Anita Graser]

On Mon, Feb 24, 2014 at 1:35 PM, Leyan <ouyang.leyan.ml at hotmail.com> wrote:
> On 02/24/2014 05:45 PM, Marco Hugentobler wrote:
>>> - GUI: have a tab in project properties where non-default datum
>>> transforms would be managed - instead of requiring the user to select
>>> the datum transform immediately when the layer was added (and without
>>> being able to change the decision later)
>>
>> I don't have a strong opinion if it should be in project properties or if
>> the user should be asked immediately. The immediate selection has been
>> implemented following the behaviour of a commercial GIS. However, the
>> possibility to change it later easily does also sound good to me.
>>
>> Btw., congratulations to the merge of the multithreading branch!
>>
>> Regards,
>> Marco
>
> Hi Marco,
>
> I struggled quite a lot with the interface of a very famous commercial GIS
> on this issue, I believe there is a real possibility to do better here ! I
> think it is essential to allow the user to change the transformation later
> easily, maybe somewhere linked from the CRS choice ? That's where I would go
> as a user.
>
> I was also wondering how you see the relationship between CRS and datum
> transformation. I use CRSs (all the Beijing54 CRS) which already have a
> +towgs84 defined in their definition, but several towgs84 coefficients are
> offered when loading the layer. It is confusing to have to answer that, and
> if something else than the default coefficients is chosen, the result is
> incompatible with the proj4 definition. Should we strip these CRS
> definitions of the towgs84 coefficients (which were only meant for a small
> area initially, so are not valid everywhere)?

Please note the following related issue:
http://hub.qgis.org/issues/9396 "deactivate the Select datum
transformations dialog by default"

Maybe the ticket has to be reopened?

Best wishes,
Anita