[Qgis-user] all prj-files broken?

[Robert Nuske]

Hi Bernhard,

> I reread your former mail and looked at the attached prj files. For my
> area (now EPSG:31468 (Gauss-Krüger, Potsdam, Bessel), future EPSG:25832
> (ETRS89, zone 32)) I experimented with proj4 last year.
> I found that the definition of EPSG:25832 in proj4 contains no datum
> information, so datum is "D_unknown". Therefore transforming from
> EPSG:31468 to EPSG:25832 using cs2cs does not perform a datum shift.

This might be true for proj4 but definitely not for ogr2ogr. 

ogr2ogr applies some magic to recognize the datum and then produces more 
accurate reprojections. Some test cases (EPSG:31468 -> EPSG:25832) showed a 
shift of about 168m with Datum "D_unknown" and only 2m with Datum 
"D_Deutsches_Hauptdreiecksnetz". A shift of 2m is acceptable for my 
application but not 168m.

Even better results were obtained by reprojecting with ogr2ogr using -s_srs 
and -t_srs or within QGIS. More details in a personal mail.



I am still missing badly valid datum parameters in prj-files written by QGIS



cheers,
  robert



> Am 15.09.2011 17:45, schrieb Robert Nuske:
> > Hi List,
> > 
> > are all prj-files written by QGIS broken or just mine?
> > 
> > 
> > Saving a layer as shapefile writes always a prj-file with an unkown
> > datum.
> > 
> > That causes problems visualising that shapefile in other GIS and
> > reprojecting it  outside of QGIS and. I tested different projected
> > coordinate systems with QGIS 1.7 from ubuntugis_unstable on Ubuntu
> > 10.10.
> > 
> > 
> > 
> > Example:
> > ogrinfo -al -so utm32.shp
> > INFO: Open of `utm32.shp'
> > 
> >        using driver `ESRI Shapefile' successful.
> > 
> > Layer name: utm32
> > Geometry: Point
> > Feature Count: 3
> > Extent: (-3.751456, -2.466990) - (4.628155, 1.957282)
> > Layer SRS WKT:
> > PROJCS["UTM_Zone_32_Northern_Hemisphere",
> > 
> >      GEOGCS["GCS_GRS 1980(IUGG, 1980)",
> >      
> >          DATUM["unknown",
> >          
> >              SPHEROID["GRS80",6378137,298.257222101]],
> >          
> >          PRIMEM["Greenwich",0],
> >          UNIT["Degree",0.017453292519943295]],
> >      
> >      PROJECTION["Transverse_Mercator"],
> >      PARAMETER["latitude_of_origin",0],
> >      PARAMETER["central_meridian",9],
> >      PARAMETER["scale_factor",0.9996],
> >      PARAMETER["false_easting",500000],
> >      PARAMETER["false_northing",0],
> >      UNIT["Meter",1]]
> > 
> > id: Integer (10.0)
> > 
> > 
> > 
> > cheers,
> > 
> >    robert
> > 
> > P.S. I wrote a similar email a week ago, but that one was probably too
> > long.
> > 
> > 
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--
   Robert Nuske
   Department of Ecoinformatics,
   Biometrics & Forest Growth

   Buesgenweg 4
   37077 Goettingen
   GERMANY

   Phone: +49-551-39-22362
   Fax  : +49-551-39-3465
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