[R-sig-Geo] convert Map shapes to logical matrices - for setoperations

[Roger Bivand]

On Tue, 7 Dec 2004, James Muller wrote:

> Hi, I have a little problem.  (and sorry for the cross-post of this)
> 
> I'm trying to do the following:
>  Convert  _projected_ shapes from a Map object into logical matrices. 
> That is, rasterize a shape into a logical "in-the-shape" and 
> "out-of-the-shape" matrix.
> 
> What I'm trying to do is get an 'equal-area' estimate of the area of 
> intersection between two overlapping shapes, without any concern about 
> there being holes in the shapes. I know owin can deal with most of this, 
> but for sure it doesn't support geographic projections, which will allow 
> the 'equal-area' estimate to be equal.
> 
> An idea I had was to pipe the output of the maptools Map plotting 
> function map into the matrix. But I really have no idea how to do that 
> at this point in my R education.

Do you have Shapes from the Map object in lat-long or projected 
coordinates? If they are in projected coordinates, you can generate a grid 
with the standard function expand.grid() and then use a point-in-polygon 
function, such as inside.owin() or similar, on each of the grid points. If 
your shape polygons are in geographical coordinates, then consider 
projecting them, see http://www.remotesensing.org/proj for open source 
projection software. You would then be looking for grid points in A, and 
among those, grid points in B, I think. I don't think I'd want to have to 
generate equal area grid points in lat-long, though you could do that 
using proj inversely.

Roger

> 
> Any suggestions are extremely welcome.
> 
> Cheers,
> 
> James
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: Roger.Bivand at nhh.no