[Qgis-developer] Datum transformations

Leyan ouyang.leyan.ml at hotmail.com
Mon Feb 24 04:35:00 PST 2014

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)?

Also, do you see a need to allow to define a custom datum 
transformation, or do you think it is enough that user can create a 
custom CRS with different towgs84 parameters?



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