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 <caron at 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 Carr?e)
>>>
>>> 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, ?p for example.
>>>
>>> (b) The geographic longitude in degrees of the southern pole of
>>> the coordinate system, ?p 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 ?p degrees about the geographic polar
>>> axis, and then rotating through (90 + ?p) 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 at cgd.ucar.edu
>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>>
>>
>
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Received on Wed Mar 06 2013 - 12:32:37 GMT
