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 <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
