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

Davide Cesari dcesari at arpa.emr.it
Tue Aug 14 04:49:15 EST 2012

```Hello Martin, in principle this should be a job for proj's ob_tran
transformation

ob_tran : General Oblique Transformation
Misc Sph
o_proj= plus parameters for projection
o_lat_p= o_lon_p= (new pole) or
o_alpha= o_lon_c= o_lat_c= or
o_lon_1= o_lat_1= o_lon_2= o_lat_2=

but I am afraid that the way the parameters are to be fed to proj is
different from the convention used in meteorological gridded data that I
suppose you are using, there was some past discussion here:

http://lists.maptools.org/pipermail/proj/2010-June/005234.html
http://lists.maptools.org/pipermail/proj/2011-October/005913.html

please report on any success. Anyway the idea proposed by Mikael may be
a good workaround which is worth tesating,
Best regards, Davide

--
============================= Davide Cesari ============================
Servizio IdroMeteoClima - ARPA Emilia Romagna
NWP modelling - Modellistica numerica previsionale
Address:   ARPA-SIM, Viale Silvani 6, 40122 Bologna, Italy
========================================================================

On 13/08/2012 22:40, Mikael Rittri wrote:
> 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 =0, lon = 0 on the rotated
> grid. First, project to an auxiliary stereographic grid:
...
>
> 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_LAT9.25 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
>>
>> _______________________________________________
>> Proj mailing list
>> Proj at lists.maptools.org
>> http://lists.maptools.org/mailman/listinfo/proj
>

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