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. > > > > > > ________ Information from NOD32 ________ > > This message was checked by NOD32 Antivirus System for Linux Mail Server. > > http://www.nod32.com > > > > > > > > > > _______________________________________________ > > Qgis-user mailing list > > [email protected] > > http://lists.osgeo.org/mailman/listinfo/qgis-user > > > > > > ________ Information from NOD32 ________ > > This message was checked by NOD32 Antivirus System for Linux Mail Server. > > http://www.nod32.com > > ________ Information from NOD32 ________ > This message was checked by NOD32 Antivirus System for Linux Mail Server. > http://www.nod32.com -- 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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