[Qgis-developer] Datum Transformation - parameters for mainland Portugal

[Andre Joost]

Hi Pedro,

Am 18.06.2014 15:13, schrieb Pedro Venâncio:

> I have an update to the parameters that are in Datum Transformation
> for mainland Portugal.
>
> After an analysis of the parameters that are currently inserted in
> srs.db, I think we should keep only the latest parameters that are
> provided by the Portuguese Surveying Authority (Directorate-General
> of the Territory), to avoid confusion among users.
>
> So, we should keep only the transformation parameters of Molodensky,
> Bursa-Wolf, and two grids NTv2 that are in use here in Portugal, one
> from the Directorate-General of the Territory, and another from the
> University of Porto.
>

it has little use to change the parameters in QGIS only.

All applications that use GDAL/PROJ use the same database of CRS. This 
is kept in sync with the official EPSG database at 
<http://www.epsg-registry.org/>

The EPSG database offers different transformations from local datums to 
WGS84. One of these is usually bundled to the coordinate system when you 
use QGIS/GDAL/PROJ. Other software like ARCGIS keeps coordinate systems 
and datum transformations separated.

Latest versions of QGIS allow to use a different datum transformation, 
but keep in mind that software that stores only EPSG codes (like 
spatialite) will use nethertheless the default datum transformation.

If you feel uncomfortable with the default datum transformation, and you 
have "official" sources, you might open a ticket for the CRS databases 
used in GDAL and PROJ. The parameters should be at least in the EPSG 
database.
QGIS syncs with that database after installation to avoid discrepancies 
on your own computer.

Greetings,
André Joost