[Proj] forward/inverse projection logic for ob_tran

Davide Cesari dcesari at arpa.emr.it
Mon Oct 10 02:54:14 EST 2011

Il -10/01/-28163 20:59, Hermann Peifer ha scritto:
> 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

Hi Hermann, thank you the message, I had also raised the problem some 
time ago, see e.g. this message in the proj mail archive:


but there was not much reaction, I think the problem is simply that the 
use of this kind of rotation in Numerical Weather Prediction models and 
in Cartography is the opposite. Maybe an additional set of switches 
could be added to the ob_tran transform in order to get the expected (by 
us) behavior, unfortunately I have no time to dedicate to that at the 
moment, but I would like to do it if there is a chance that it will be 
accepted in the main proj code.
	best regards, Davide

============================= Davide Cesari ============================
Servizio IdroMeteoClima - ARPA Emilia Romagna
NWP modelling - Modellistica numerica previsionale
  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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