[Qgis-developer] Datum Transformation with target_crs_code different than 4326 gives huge errors

[Andre Joost]

Am 04.07.2014 23:07, schrieb Pedro Venâncio:
> Hi,
>
> I've been doing some tests and found that, using a different
> target_crs_code than 4326 (wgs84), Datum Transformation simply
> ignores the values ​​of transformation.
>

>
> The error we get is huge, and I get exactly the same error if I set
> the shift values all to zero. It simply ignores the transformation
> using a different target_crs than 4326. The exception are NTv2 grids.
> Assigning them different target_crs, work well (there is one with
> target_crs 4150 and seven with 4258).
>
> Actually it's even worse than ignoring, because if I do not choose
> any transformation and do "Cancel" on the Datum Transformation
> window, the point is reprojected based on the values ​​of default CRS
> +towgs84 parameter (gives an error of about 1.13m). With this data I
> am testing (EPSG:3763 vs. EPSG:27493), even without using on-the-fly
> reprojection, the difference between the point is only about 3.2m.
> Using target_crs 4258 in srs.db, regardless of the values​​, the
> error is more than 123m.
>
> Maybe it generate some conflict with +towgs84 parameter?
>
> The most recent transformations in some European countries are
> calculated for 4258 and with this issue we can not use them in QGIS.
>
> I've opened a ticket here [0], where I put test data and images of
> the results.
>
>

I think it should be save to set the target_CRS in the QGIS Datum 
transformation tool internally to 4326 if 4258 is provided, or fill in 
the tfm values directly to target_CRS 4326 (although the EPSG database 
has 4258). Nadgrids work differently, but could be handled the same way. 
WGS84 and ETRS89 are still considered as identical.

Greetings,
André Joost