[Qgis-user] Re: Annoying CRS-Problem with EPSG 31468 / 2167

[Andre Joost]

Am 22.03.12 09:28, schrieb bernhard.stroebl at jena.de:
> Problem confirmed here
>
> Example of prj file (produced by GRASS):
> PROJCS["Transverse_Mercator",GEOGCS["GCS_bessel",DATUM["D_Deutsches_Hauptdreiecksnetz",SPHEROID["Bessel_1841",6377397.155,299.1528128]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",12],PARAMETER["scale_factor",1],PARAMETER["false_easting",4500000],PARAMETER["false_northing",0],UNIT["Meter",1]]
>
>
> QGIS loads this with EPSG:2167 which contains the following proj4
> parameters:
> +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=krass
> +towgs84=26,-121,-78,0,0,0,0 +units=m +no_defs
>
> whereas EPSG:31468 has:
> +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=bessel
> +towgs84=582,105,414,1.04,0.35,-3.08,8.3 +units=m +no_defs
>
> The problem may be there are no explicit towgs84 parameters BUT there is
> a datum given in the prj file which other software translates into a
> towgs84 parameter and vice versa, QGIS does not.

... and Qgis has picked another ellipsoid (Krassowsky instead of Bessel, 
which is definitely wrong.

I had similiar problems when transformning Geotiff raster data from DHDN 
to Google Mercator using gdalwarp: EPSG:900913 always gave an offset to 
the north by 20km, while using EPSG:3785 was placed correctly.

By the way, the towgs84 paramters for DHDN are only a rough geuss. Exact 
values differ from region to region, or can be replaced by the 
high-accuracy beta2007 transformation.

Greetings,
Andre Joost