[Qgis-developer] Datum transformations

[Leyan]

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