[Qgis-developer] Datum transformations

[Marco Hugentobler]

Hi Leyan

 >I think it would be better to remove the towgs84 parameters for the 
ellipsoids where there is no clear choice, and offer to set the datum 
transformation, but I agree it is hard to >go about it in a 
user-friendly way.

Yes, it was the original behaviour to always ask the user, however it 
was considered as not user friendly.

 >In my case, the coefficients offered for Beijing54 ellipsoid come 
from  different sources and only cover a small area each. This ellipsoid 
is only used in China and there is no >official list of coefficients to 
transform (CRS are kind of an important secret in China, which does not 
really make sense ...).

Even in Switzerland, where it seems to be a clear choice (one CRS for 
the whole country), the default towgs84 parameters of the national crs 
are only accurate one digit behind the dot. So choosing the 
transformation with an accuracy of three digits behind the dot in the 
datum transform dialog makes a well visible difference.


Regards,
Marco




On 25.02.2014 12:56, Leyan wrote:
> On 02/24/2014 09:16 PM, Marco Hugentobler wrote:
>> 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 :-)
> That would be such a coincidence ;-)
>>
>> 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?
> I think it would be better to remove the towgs84 parameters for the 
> ellipsoids where there is no clear choice, and offer to set the datum 
> transformation, but I agree it is hard to go about it in a 
> user-friendly way.
>
> In my case, the coefficients offered for Beijing54 ellipsoid come 
> from  different sources and only cover a small area each. This 
> ellipsoid is only used in China and there is no official list of 
> coefficients to transform (CRS are kind of an important secret in 
> China, which does not really make sense ...).
>>
>> >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.
>>
> I was thinking of improving the custom CRS entry further by splitting 
> the proj4 definition in a gui. In that case, datum transformation 
> could be a part of it, or near it. I agree this will not be used by 
> most users, but still, it would be better for the few end users who 
> need it. Anyway, I don't have a lot of time to code so my progress on 
> these things is very slow ...
>
>> Regards,
>> Marco
> 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