Due to the current gap in continued funding from the U.S. National Science Foundation (NSF), the NSF Unidata Program Center has temporarily paused most operations. See NSF Unidata Pause in Most Operations for details.
Due to the current gap in continued funding from the U.S. National Science Foundation (NSF), the NSF Unidata Program Center has temporarily paused most operations. See NSF Unidata Pause in Most Operations for details.
Hello Andrea :-)I did an analysis of "Rotated Pole" implementations in UCAR library and PROJ a few months ago for trying to reverse engineering their mathematical definitions. It was an attempt (still in progress) to get a formal definition of this operation method in a way similar than what EPSG does. My current understanding of the situation is documented there:
https://github.com/opengeospatial/MetOceanDWG/blob/main/MetOceanDWG%20Projects/Authority%20Codes%20for%20CRS/Pole%20rotation.mdIn summary, while CF-convention cites only one pole rotation method (namely "rotated_latitude_longitude"), the UCAR netCDF library has two implementations: the above cited one and "rotated_latlon_grib". The source code of those two implementations look totally different, but they are really the same thing computed in different ways. The major difference is that "rotated_latitude_longitude" rotates the /North/ pole while "rotated_latlon_grib" rotates the /South/ pole. Rotating the wrong pole causes an error of 180° in longitude and 90° - φ (or something like that, I do not remember exactly) in latitude, which looks like what you are observing with Africa. The method defined by CF-Convention rotates the North pole, while the method defined by World Meteorological Organization (WMO) used in GRIB files rotates the South pole.
Note: looking at the math, it appears that we do not need two distinct implementations for the North pole and South pole cases. We can use the South pole rotation (the one defined by WMO) as the fundamental definition, and express the Norh pole case (the one defined by CF-Conventions) with a transformation applied on the input parameters before to delegate to the formulas of the South pole case. The above link gives examples for testing the same coordinate operations with UCAR, PROJ and Apache SIS libraries.
Martin
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