[R-sig-Geo] spTransform: unable to convert coordinates

[Martin Ivanov]

Dear Roger,

Thank You very much for Your responsiveness. I tested Your new revision 
with points, data frames and pixels and
everything seems to be ok. It is now only this annoying 
counter-intuitive discrepancy between source and target proj,
but it is not really an obstacle. It seems to come from proj4 itself. 
There one has to use proj to get to the unrotated coordinates from the
rotated, and invproj to get the rotated from the unrotated ones. See 
http://lists.maptools.org/pipermail/proj/2011-October/005906.html.

I would also like to answer Your question from Your first reply, if such 
transformations with rotated grids are often encountered.
In Meteorology, especially when it come to regional weather and climate 
models, the calculations are performed in a grid,
suitably adjusted for the region of interest, so that the equator of the 
grid passes through the region. Thus the
distortions from the planar representation are minimized. So all the 
data coordinates have to be first transformed to that rotated grid.
And then, when the calculations are over, one has to switch back to the 
normal coordinates to make plots ready to use.
You could take a look for example at the following document
http://cordex.dmi.dk/joomla/images/CORDEX/cordex_archive_specifications_120720.pdf
to get an idea.

Concerning the use_ob_tran parameter, yes, it is a solution. Yet I am 
not able to think of a situation where one would need it set to FALSE,
but probably people with more experience and from other disciplines can 
contradict me.

Thank You very much once again for the support.

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