Default "skew angle" for CF grid mapping "oblique mercator" seems wrong

[aulemahal]

Hi!

When using pyproj.CRS.from_cf on attributes defining an oblique mercator grid mapping, it seems that the Pyproj projection if rotated 90° from the expected output. Specifically, I think this line should assign 90, not 0 :

pyproj/pyproj/crs/_cf1x8.py

Line 237 in b7b8384

angle_from_rectified_to_skew_grid=0.0,

I am plotting data from a climate model that runs on the oblique mercator projection. In this first plot, the cartopy CRS was obtained with : cartopy.crs.Projection(pyproj.CRS.from_cf(oblique_mercator)):

In this second plot, I instead used cartopy's implementation of the same, cartopy.crs.ObliqueMercator :

We can clearly see that the second figure is what we expected.

The grid mapping variable has the following attributes:

{'grid_mapping_name': 'oblique_mercator',
 'azimuth_of_central_line': 90.0,
 'latitude_of_projection_origin': 46.0,
 'longitude_of_projection_origin': 263.0,
 'scale_factor_at_projection_origin': 1.0,
 'false_easting': 0.0,
 'false_northing': 0.0}

The CF conventions do not define the "Angle from Rectified to Skew Grid". In cartopy, it is not specified, so the default is used. In pyproj, it is specified as 0°. From, this page, I gather that the default value should be 90°. And indeed, we are seeing a 90° degree rotation when using pyproj's projection object.

Thus why I think the value in pyproj is wrong.

[snowman2]

Thanks for the helpful explanation. A PR with the change is welcome.