[Proj] proj4 specifications for a rotated latlon grid on a sphere?

[Mikael Rittri]

Hello Martin. 
In some GIS software, it would indeed be possible to rotate
a graticule via +towgs84 parameters with huge rotations.
But I think Proj.4 (like most GIS software) uses a simplified,
linearized implementation of the rotations, assuming they
are small (up to 5 or perhaps 10 arc seconds).

But you can rotate a graticule on a sphere by using Proj.4
twice. Let's try it for the point lat = 80, lon = 0 on the rotated
grid. First, project to an auxiliary stereographic grid:

>proj +proj=stere +lon_0=0 +lat_0=90 +a=6371000 +b=6371000

0    80

0.00    -1114780.55

Next, inverse project back to long/lat using a _different_
instance of stereographic:

>proj +proj=stere -I +lon_0=-162 +lat_0=39.25 +a=6371000 +b=6371000

0.00    -1114780.55

162dW    29d15'N

That's the corresponding long/lat on the plain graticule. 
(I haven't tested if this works on or near the south poles
 of the graticules.)

I have assumed that the prime meridian (zero longitude) of
the rotated graticule goes south from the rotated north pole.
If it doesn't, you can model its direction by changing the
value of lon_0 to something non-zero in the first definition. 

It may be possible to chain the forward and inverse
projection together into a single command line by using
a pipe, but I don't remember the syntax for that. 

Best regards

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

13 aug 2012 kl. 17:19 skrev "Martin Ivanov" <martin.ivanov at ifg.uni-tuebingen.de>:

> Dear PROJ4 users,
> 
> I have a rotated latitude-longitude grid with coordinates of the north 
> pole POLE_LAT=39.25 and POLE_LON=-162.
> The data are on a sphere with radius 6371 km. Is it possible to define 
> proj4 specifications for this grid, so that I can make
> coordinate transformations? Most interesting for me would be how to 
> specify the towgs84 parameter.
> 
> Any suggestions will be appreciated.
> 
> Best regards,
> 
> -- 
> Dr. Martin Ivanov
> Eberhard-Karls-Universität Tübingen
> Mathematisch-Naturwissenschaftliche Fakultät
> Fachbereich Geowissenschaften
> Water & Earth System Science (WESS)
> Hölderlinstraße 12, 72074 Tübingen, Deutschland
> Tel. +4970712974213
> 
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