[Qgis-user] Projection not recognised EPSG:31259

Helmut Kudrnovsky hellik at web.de
Tue Mar 24 11:15:41 EDT 2009


hi all,

some comments in the text

>Hello,
>
>from Roland's (he was the person who brought first light for me in this 
>issue) and my/our experience (we work together, sometimes ;-) the 
>situation presents itself like that:
>
>* If you have an ESRI-generated .prj-file, the towgs84 - parameter is 
>missing: Programms using GDAL/Proj & Co. (like QGIS) need this parameter 
>to be able to make projections on the fly. So, you have to edit the .prj 
>file in question, or use the suitable one from the ones I sent earlier 
>today, and name it accordingly  - if you make the jump from ESRI to QGIs 
>for example
>
>* If you stay just with ESRI, it is no problem, as far as I can tell, 
>ArcGIS does projections on the fly within its programming logic, it 
>seems (just a wild guess)

in my work with ESRI ArcGIS 9.1 through 9.3 and my data with projections in wgs84, MGI, Monte Mario etc. 
i have to define manually for every projection the towgs-parameter that the on-the-fly-projection of all data
together is correct. 

if you don't do this, there is for example a north-east-shift of about 70m in the on-the-fly-projection 
of Austrian and wgs84-data. 

So the situation is similar to QGis and other GDAL/Proj4-based GIS-software. And
maybe for your interest, since version 9.2 ArcGIS uses also GDAL.

>
>* As a conclusion: The .prj files are not really wrong, they are 
>incomplete (Is there an official standard/definition somewhere?). To be 
>on the safe side, make your .prj-files complete, because if ESRI 
>stumbles over the towgs84-parameter it leaves it alone, so jumping back 
>and forth is no problem...

I think the prj-file missing the towgs-parameter is not wrong, it's maybe
the official standard?

just try ogr2ogr with a projection-warp, in the 
prj-file of the new shapefile there is no towgs-parameter included.

>
>* From a kind of "scientific" point of view: If you have the parameter 
>in your .prj-files, it is documented what you intended to do with the 
>data...
>

that's sounds good, but be aware that the towgs-parameter can differ, also in Austria. So you can order from
Bundesamt für Eich- und Vermessungswesen (http://www.bev.gv.at/) for Austria adopted towgs-parameter for
10 km x 10 km-grids with better accuracy than the epsg-towgs-data for the Austrian MGI.

In my experience with Austrian govermental gis-data in the last years, beeing carefull and check all data saves 
nerves ;o) ...

best regards
Helli



>Maybe the situation is a little clearer now...
>
>best wishes,
>
>Albin


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