Hi Burkhardt:
Of course we cant change the existing conventions as there are files
that already use it. But we need to document it so that we know when we
are doing it right.
Im cleaning up my code and noticed that i have two implementations, one
for grib and one for CF. So Id like to merge the two.
John
On 3/6/2013 12:32 PM, Burkhardt Rockel wrote:
> 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 ??
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>> 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 - 14:32:52 GMT
