QGIS » QGIS Application

Antonio Sobral Almeida wrote:

Hello Giovanni
Thanks for your update.
Yes, EPSG 27493 and 20790 uses the DATUM transformation parameters that are lacking on the above mentioned CRS's.
The problem arises when you have shapefiles previously created or edited on ESRI ArcGIS, with projection files (.PRJ) that do not have these CRS’s but, instead, the ESRI-based CRS’s (Lisboa_Hayford_Gauss_IGeoE, Lisboa_Hayford_Gauss_IPCC,
Datum_73_Hayford_Gauss_IGeoE or Datum_73_Hayford_Gauss_IPCC), which are widely used in Portugal.
When QGIS reads these ESRI projection files (and, fortunately, QGIS can read ESRI projection files), QGIS do not find any DATUM transformation parameters, and simply QGIS reproject the shapefile WITHOUT ANY DATUM SHIFT. This makes the reprojected shapefile being ALLWAYS displaced, around 200-300 meters, from the right place, where it should be if there were the transformation parameters.
That simply means that GIS users working with shapefiles created with ESRI GIS, using ESRI-based CRS’s, will have to update the CRS each time a shapefile is loaded in QGIS, simply because QGIS database is not yet corrected!
We sincerely hope that QGIS make this very simple correction to its database, allowing GIS users working in Portugal to move to QGIS, specially to its top-notch 2.0 version!

Hi Antonio,
yes I know what you mean, "pois" I live and work in Portugal.

Just to let me understand better:

I remember that ESRI sofware was not saving vectors with towgs84 parameters in their Portuguese CRSs definitions (102160/1/4/5).

Are you saying that they do now?

If the answer is "yes" then you may want to consider that if the QGIS definitions will be updated, then all old the vectors that were been given the definitions without the towgs84 parameters will be given (on load) by QGIS an internal code (it starts from 100000), so then there will be a lot of people complaining that their shapes are given a "strange" CRS.

Meanwhile you may want to consider manually giving to your vectors the 20790/27493 and then saving copies in the same CRS.

Feel free to contact me privately: [email protected]

Hello Giovanni,

No, ESRI projection files do not have ToWGS84 parameters.

This and the fact that the above mentioned ESRI-based projection files are widely used in Portugal, makes any migration intention to QGis rather improbable, due to the reasons stated above.

Antonio Sobral Almeida wrote:

Hello Giovanni,

No, ESRI projection files do not have ToWGS84 parameters.

This and the fact that the above mentioned ESRI-based projection files are widely used in Portugal, makes any migration intention to QGis rather improbable, due to the reasons stated above.

Hi Antonio,

I would not say that migrating to QGIS in Portugal is "rather improbable", considered the many (many) persons and entities that have happily migrated.

The issue you are raising is easily solved, I just lack the time to answer extensively right now. It is possible that things can be improved but certainly is not a blocker issue.

Write to you soon.

Hi Giovanni,

I would not say that migrating to QGIS in Portugal is "rather improbable", considered the many (many) persons and entities that have happily migrated.

That is a VERY GOOD reason for QGis developers to correct this bug!

Regards
Antonio

Antonio Sobral Almeida wrote:

Hi Giovanni,

I would not say that migrating to QGIS in Portugal is "rather improbable", considered the many (many) persons and entities that have happily migrated.

That is a VERY GOOD reason for QGis developers to correct this bug!

Regards
Antonio

Dear Antonio,

the suggestion to add the towgs84 parameters to the ESRI CRSs makes sense, anyway this cannot be seen strictly as a bug.

Until the CRS will be modified you can easily workaround the problem in many ways:

+) when adding the vectors manualy change their CRS to the equivalent EPSG code (20790/20791/27493)

+) remove the .prj/qpj files of your layers and configure qgis (in the general options) to give them (on load) a specific CRS (20790/20791/27493)

+) create copies of your layers giving them the EPSG CRSs (20790/20791/27493): it is not necessary to make this operation one by one, you can batch process your layers using the processing toolbox (aka Sextante) and the tools "reproject layer" (for vectors) and "warp" (for rasters).