[R-sig-Geo] custom projection (SpatialPolygon)

[Roger Bivand]

On Thu, 7 Aug 2008, Neil Davis wrote:

> I have a dataset which uses a custom projection, and I would like to import 
> shape files to add maps to the plots.  I was able to import the shape file 
> using maptools as both a SpatialLine, and SpatialPolygon, however I'm not 
> sure how I can modify the coordinates to my custom map projection.  I have 
> converted the standard R maps to my map projection, but the spatial objects 
> are a little out of my understanding, as I am not sure how to get the 
> coordinate values out of and then back into the spatial objects.
>
> Does anyone know how I might be able to modify the coordinates of the spatial 
> objects to my projection.

If you cannot design a custom PROJ.4 description (especially the x and y 
offsets may be helpful) and use spTransform() methods in rgdal, you could 
look at elide() methods in the maptools package, which do allow some 
offsetting and perhaps even rotation (you need the angle) and some 
scaling. But the you lose the portability of a PROJ.4 description.

Hope this helps,

Roger

>
> Thanks,
>
> Neil Davis
> Research Associate
> Institute for the Environment - UNC Chapel Hill
>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no