[Proj] Representing rotated poles

support.mn at elisanet.fi support.mn at elisanet.fi
Sun Jan 27 23:53:27 EST 2008


what I understand, the most easy way is simply to rotate the poles back where they should be before using any proj-4, since that assumes that your coordinates are about right, datum shifts can only handle small shitfs in the rang of few hundred meters usually. I think proj-4 uses pole shifting internally with some projections, but externally nobady knows, since they are rotated back to their places after the math... well, several projections do that kind of tricks.. (?)

So you simply do the same. Add few lines in your code so that the poles get back to their right places, and then call proj-4. And if your application needs the results to be rotated, you simply add some more lines of code.

Regards: Janne.


"Pascoe, S (Stephen)" <S.Pascoe at rl.ac.uk> kirjoitti: 
> Hi,
> I deal with climate model data and have regional model data defined on a
> long/lat grid with the poles rotated away from their true position (in
> order to keep the pole singularities away from the region of interest).
> Typically this grid would be defined as follows:
> grid_north_pole_latitude:  39.2 deg (true latitude of rotated north
> pole)
> grid_north_pole_longitude: 198.0 deg (true longitude of rotated north
> pole)
> north_pole_grid_longitude: 0 deg (longitude of the prime meridian in the
> rotated coordinate system)
> How would I represent this coordinate system with proj4 parameters?  I
> have been trying to use +towgs84:
> +proj=longlat
> +towgs84=0,0,0,-grid_north_pole_longitude*60*60,90-grid_north_pole_latit
> ude*60*60,north_pole_grid_longitude,0
> So far this doesn't seem to work (I'm using gdal to transform an image
> to EPSG:4326).  Am I on the right lines?  Standard GIS terminology such
> as datum, spheroid etc. are unfortunately not part of the standard
> vocabulary of climate modellers.
> Many thanks,
> Stephen.
> ---
> Stephen Pascoe  +44 (0)1235 445980
> British Atmospheric Data Centre
> Rutherford Appleton Laboratory
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