convert Map shapes to logical matrices - for setoperations
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.
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