[Proj] forward/inverse projection logic for ob_tran

[Hermann Peifer]

Hi,

I am currently dealing with regional climate models, where the data 
comes in form of "rotated pole lat/lon grids", with the North Pole at 
39.25N, 162W.

I am able to convert "rotated pole coordinates" to "normal lat/lon 
coordinates" by using:

$ proj -f "%.6f" -m 57.2957795130823 +proj=ob_tran +o_proj=latlon 
+o_lon_p=-162 +o_lat_p=39.25 +lon_0=180

It looks to me that the proj/invproj (i.e. forward/inverse) projection 
logic is opposite to what I would expect: I need to run proj (i.e. the 
forward projection) in order get from the "rotated lat/lon" coordinates 
(say: lon=0,lat=0) back to the "normal lat/lon coordinates" (here: 
lon=18.0,lat=50.75), and vice versa.

Is this expected behaviour? If yes: what am I missing?

Thanks for your time, Hermann