[Qgis-developer] Datum transformations

[Marco Hugentobler]

Hi Leyan

 >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.

He, he, I bet it is the same GIS :-)

 >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)?

Do you mean we should strip the towgs84 parameters from 
tbl_srs.parameters or do you mean to deprecate/remove entries from 
tbl_datum_transform?

 >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?

It is possible to add new entries to tbl_datum_transform to create 
custom datum transformations. Yes, it would be more convenient to have a 
gui for that, even if I don't know how often it will be used. You might  
judge it better since you obviously have to deal often with CRS issues.

Regards,
Marco


On 24.02.2014 13:35, Leyan 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)?
>
> 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?
>
> Regards,
>
> Leyan
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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
Technical Advisor QGIS Project Steering Committee