[Proj] rotated pole ob_tran help needed!

Davide Cesari dcesari at arpa.emr.it
Mon Jun 7 10:04:31 EST 2010

Haileye wrote

> Hello All,
> I am trying to convert rotated pole coordinates to regular lat & lon coordinates. I have both coordinates, but am trying to get the right proj4 definition (or at least close). The rotated pole projection coordinates are defined as yc & xc, the projection description is described below:
>         double yc(yc) ;
>                 yc:axis = "Y" ;
>                 yc:standard_name = "grid_latitude" ;
>                 yc:long_name = "latitude in rotated grid" ;
>                 yc:units = "degrees" ;
>         double xc(xc) ;
>                 xc:axis = "X" ;
>                 xc:standard_name = "grid_longitude" ;
>                 xc:long_name = "longitude in rotated pole grid" ;
>                 xc:units = "degrees" ;
>         char rotated_pole ;
>                 rotated_pole:grid_mapping_name = "rotated_latitude_longitude" ;
>                 rotated_pole:grid_north_pole_latitude = 42.5 ;
>                 rotated_pole:grid_north_pole_longitude = 83. ;

Hi, I am quite new to proj and trying to use it probably for the same 
purpose as indicated here (processing of Numerical Weather Prediction 
model data on rotated grids), so I come back to this old thread. This is 
the link to the archive, in case it has been forgotten:


I can reproduce haileye's result with the following command line 
parameters to proj:

invproj -f "%f" -m 57.295779506 +proj=ob_tran +o_proj=latlon +o_lon_p=83 
+o_lat_p=42.5 +lon_0=180
in> -127.163970947 12.5568675995
out> -33.880011	-28.379995

then the opposite transformation:

proj -f "%f" -m 57.295779506 +proj=ob_tran +o_proj=latlon +o_lon_p=83 
+o_lat_p=42.5 +lon_0=180
in> -33.880011	-28.379995
out> -127.163971	12.556868

-m 57.295779506 is needed for getting output or providing input in 
degrees, otherwise it is radians.
+lon_0=180 is required otherwise there is a shift of 180 degrees, 
probably because the transformation is underdetermined with only the 
given parameters?!

 From what I could understand, the point here is that the interpretation 
of pole coordinates is the opposite in the GIS/Cartography community and 
in the Numerical Weather Prediction community: in the first case they 
are the coordinates of the Earth NP in the map system, while in the 
second case they are the geographical coordinates of the rotated NP. For 
this reason the transformations are used in the opposite way as designed.
It would be nice to hear the opinion from some proj guru on whether this 
is a correct use, or a different approach should be used. I enclose the 
definition of "rotated grid" from the World Meteorological Organization" 
document, here Southern Pole is used and point (c), angle of rotation 
should be 0 in the examples examined here.

=== WMO specification ===
(6) 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.
=== end WMO specification ===

Thank you very much for your attention, bye, Davide

============================= Davide Cesari ============================
Servizio IdroMeteorologico ARPA Emilia Romagna
Area Modellistica Numerica e Radarmeteorologia
  Phone/Fax: +39 051525926/+39 0516497501
  E-mail:    dcesari at arpa.emr.it
  Home page: http://www.webalice.it/o.drofa/davide/
  Address:   ARPA-SIM, Viale Silvani 6, 40122 Bologna, Italy

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