Lon/lat transformation

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First I should clarify I don't have previous experience with the field, so I don't know the technical terminology. My question is as follows:

I have two weather datasets:

• The first one has the regular coordinate system (I don't know if it has an specific name), ranging from -90 to 90 and -180 to 180, and the poles are at latitudes -90 and 90.
  
• In the second one, although it should correspond to the same region, I noticed something different: latitude and longitude were not the same, as they have another reference point (in the description is called a rotated grid). Together with the lat/lon pairs, comes the following information: southern pole lat: -35.00, southern pole lon: -15.00, angle: 0.0.
  

I need to transform the second pair of lon/lat to the first one. It could be as simple as add 35 to the latitudes and 15 to the longitudes, since the angle is 0 and it seems a simple shifting, but I'm not sure.

Edit: The information I have about the coordinates is the following

http://www.ecmwf.int/publications/manuals/d/gribapi/fm92/grib1/detail/grids/0/ for the first onehttp://www.ecmwf.int/publications/manuals/d/gribapi/fm92/grib1/detail/grids/10/ for the second one

Apparently, the second coordinate system is defined by a general rotation of the sphere

"One choice for these parameters is:

• The geographic latitude in degrees of the southern pole of the coordinate system, thetap for example;
  
• The geographic longitude in degrees of the southern pole of the coordinate system, lambdap for example;
  
• 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 lambdap degrees about the geographic polar axis, and then rotating through (90 + thetap) degrees so that the southern pole moved along the (previously rotated) Greenwich meridian."
  

but still I don't know how to convert this to the first one.

Any help on the subject will be much appreciated



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