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

[Marco Hugentobler]

Hi Pedro

QGIS imports the towgs84 transformations from GDAL and adds a list of 
ntv2 transformations into srs.db.
So I'm going to add/modify the ntv2 transformations (if there are no 
objections from other portuguese users). For the towgs84 ones I agree 
with Andre that it is better to bring it to gdal/epsg level.

Regards,
Marco

On 18.06.2014 16:59, Andre Joost wrote:
> 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
>
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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