Hi all, the following tries to describe how to use a rotated pole coordinate system in grass (developed with the help of Hermann Pfeiffer). Such a system is common in regional climate and weather prediction systems. Here mainly because of numerical issues, wgs84 coordinates, which are assumed to lie on a sphere, are modified such that the north pole is put on another place on the sphere (to have the equator in the region of interest). The new coordinates of the pole are called o_lat_p and o_lon_p here.
The proj4 library handles that case with the "ob_tran" projection but with opposite direction! To get normal lat lon coordinated from rotated ones, use $ proj -v -f "%.6f" -m 57.2957795130823 +proj=ob_tran +o_proj=latlon +o_lon_p=LON_POLE +o_lat_p=LAT_POLE +lon_0=180 and for the opposite direction $ invproj -v -f "%.6f" -m 57.2957795130823 +proj=ob_tran +o_proj=latlon +o_lon_p=LON_POLE +o_lat_p=LAT_POLE +lon_0=180 The factor 57.2957795130823 = 180/pi ensures the output of degree instead of radient. OGR does not support ob_tran: http://trac.osgeo.org/gdal/ticket/4285 Furthermore, I was not able to create a ob_tran location with the grass gui, I always got proj4 parse errors! Because of that and the opposite direction of the transformation, the transformation in grass is a bit tricky. Unrotated to rotated system --------------------------- * read in the data in a wgs84 latlon location * change the PERMANENT/PROJ_INFO of that location to name: General Oblique Transformation datum: wgs84 towgs84: 0.000,0.000,0.000 proj: ob_tran o_proj: latlon ellps: wgs84 a: 6378137.0000000000 es: 0.0066943800 f: 298.2572235630 lat_0: 0.0000000000 lon_0: 180.0000000000 o_lat_p: LAT_POLE o_lon_p: LON_POLE and PERMANENT/PROJ_UNITS to unit: degree units: degrees meters: .0174532925 with .0174532925 = pi/180. v.in.ogr did not work for me after changing to that projection, I guess because of ogr does not support it. * create another wgs84 latlon location (this will be the rotated system) * project the data from first system into the second Rotated to unrotated system --------------------------- * create a wgs84 latlon system and read in the data with coordinate system check disables * create another location with the PERMANENT/PROJ_INFO and PERMANENT/PROJ_UNITS from above * project from wgs84 to ob_tran system HTH, Sebastian
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