John, the rotated pole definition was requested by me in 2002 (see the thread "projections/default cell_methods"). In this thread the present description was chosen. Please note: the rotated pole grid mapping is NOT a PROJECTION!!!!! it is a TRANSFORMATION!!!! (therefore the GRIB description is misleading). This is probably the reason why you did not find it in Proj4. The a, b, c in the GRIB definition description is fine. Whether the rotated South Pole (grib) or North Pole (CF-Conventions) is used in the definition does not matter, the values can be easily converted to each other.
The rotated pole grid mapping as defined in the CF-Conventions document is already used widely as netCDF output standard for regional climate models in several international projects. It is also the standard in the IPCC-AR5 CORDEX model output for regional climate models. Therefore I am not in favor of changing the definition. However, a more precise description in the CF-Conventions would certainly be helpful for those not familiar with the rotated pole transformation. Regards Burkhardt ----------- Dr. Burkhardt Rockel Helmholtz-Zentrum Geesthacht Institute of Coastal Research / Group Regional Atmospheric Modeling Max-Planck-Strasse 1 D-21502 Geesthacht Germany Phone: +49 4152 87 1803 Fax: +49 4152 87 4 1803 Email: Burkhardt.Rockel (at) hzg.de www: http://rockel.staff.coast.hzg.de coordinates: 53.40579 N, 10.428647 E ----------- Am 06.03.2013 um 17:09 schrieb John Caron <ca...@unidata.ucar.edu>: > Hi Heiko: > > So > > grid_north_pole_longitude = > normalizeLongitude180(longitudeOfSouthernPoleInDegrees) > grid_north_pole_latitude = -1 * longitudeOfSouthernPoleInDegrees > > ? > > where does one find documentation on proj4's ob_tran routine? > > thanks, > John > > > > On 3/6/2013 8:03 AM, Heiko Klein wrote: >> Hi John, >> >> all our models use rotated_latitute_longitude. And we have a consistent way >> of translating it between grib, proj and netcdf-java, i.e. publicly >> available at: >> http://thredds.met.no/thredds/catalog/metno/proff4km/default/catalog.html >> >> >> grid_mapping_name: rotated_latitude_longitude >> grid_north_pole_longitude: 156.0 >> grid_north_pole_latitude: 23.5 >> earth_radius: 6367470.0 >> >> to proj: >> proj4: +proj=ob_tran +o_proj=longlat +lon_0=-24 +o_lat_p=23.5 +a=6367470.0 >> +no_defs >> >> from grib: >> lonRot = longitudeOfSouthernPoleInDegrees >> latRot = latitudeOfSouthernPoleInDegrees >> >> +proj=ob_tran +o_proj=longlat +lon_0=" << normalizeLongitude180(lonRot) << " >> +o_lat_p=" << (-1 * latRot); >> >> >> >> This is the set of parameters which have been useful. I'm not sure if they >> are named in a useful way. proj4's ob_tran even allows for a o_lon_p >> parameter, but this doesn't seem to be used by grib or CF. >> >> Best regards, >> >> Heiko >> >> >> >> On 2013-03-06 15:18, John Caron wrote: >>> The "Rotated Pole" projection here: >>> >>> http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/cf-conventions.html#appendix-grid-mappings >>> >>> >>> >>> Rotated pole >>> >>> grid_mapping_name = rotated_latitude_longitude >>> >>> >>> Map parameters: >>> >>> * >>> >>> grid_north_pole_latitude >>> >>> * >>> >>> grid_north_pole_longitude >>> >>> * >>> >>> north_pole_grid_longitude - This parameter is option (default is 0). >>> >>> Map coordinates: >>> >>> The rotated latitude and longitude coordinates are identified by >>> the standard_name attribute values >>> grid_latitude and grid_longitude respectively. >>> >>> >>> >>> is problematic. It has no reference to Proj.4, and is not in Snyder. >>> >>> Its likely the same as the GRIB-2 GDS Template 3.1: >>> >>> Grid Definition Template 3.1: Rotated Latitude/longitude (or >>> equidistant cylindrical, or Plate Carrée) >>> >>> Octet No. Contents >>> >>> 15-72 Same as Grid Definition Template 3.0 (see Note 1) >>> >>> 73-76 Latitude of the southern pole of projection >>> >>> 77-80 Longitude of the southern pole of >>> projection >>> >>> 81-84 Angle of rotation of projection >>> >>> 85-nn List of number of points along >>> each meridian or parallel (These octets are only present for >>> quasi-regular grids as described in Note 3) >>> >>> Notes: >>> >>> >>> (2) Three parameters define a general latitude/longitude >>> coordinate system, formed by a general rotation of the sphere. One >>> choice for these parameters is: >>> >>> (a) The geographic latitude in degrees of the southern pole of >>> the coordinate system, θp for example. >>> >>> (b) The geographic longitude in degrees of the southern pole of >>> the coordinate system, λp for example. >>> >>> (c) The angle of rotation in degrees about the new polar axis >>> (measured clockwise when looking from the southern to the northern pole) >>> of the coordinate system, assuming the new axis to have been obtained by >>> first rotating the sphere through λp degrees about the geographic polar >>> axis, and then rotating through (90 + θp) degrees so that the southern >>> pole moved along the (previously rotated) Greenwich meridian. >>> >>> >>> Except note that GRIB-2 uses "southern pole of projection". Not sure how >>> that affects the algorithm. >>> >>> The CF parameter "north_pole_grid_longitude" likely should be "Angle of >>> rotation". >>> >>> >>> >>> Anyone using this or have some insight on it ?? >>> >>> >>> >>> >>> _______________________________________________ >>> CF-metadata mailing list >>> CF-metadata@cgd.ucar.edu >>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>> >> > > _______________________________________________ > CF-metadata mailing list > CF-metadata@cgd.ucar.edu > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
