[R-sig-Geo] Triangular Tessellation of Polygon

[GREGOR Brian J]

Thank you very much. The triangulate method is exactly what I was
looking for.

Brian Gregor, P.E.
Transportation Planning Analysis Unit
Oregon Department of Transportation
Brian.J.GREGOR at odot.state.or.us
(503) 986-4120


> -----Original Message-----
> From: Roger Bivand [mailto:Roger.Bivand at nhh.no] 
> Sent: Tuesday, July 24, 2007 11:17 AM
> To: GREGOR Brian J
> Cc: r-sig-geo at stat.math.ethz.ch
> Subject: Re: [R-sig-Geo] Triangular Tessellation of Polygon
> 
> 
> On Mon, 23 Jul 2007, GREGOR Brian J wrote:
> 
> > I am developing functions to plot 3d maps using the rgl package. I'm
> > approaching the problem of plotting polygons by dividing 
> polygons into
> > triangular facets that then can be plotted as surfaces using the
> > triangles3d function. This has turned out to be somewhat cumbersome
> > using my approach which is as follows:
> > 1) Use read.shape from maptools to read in a polygon shapefile
> > 2) For each group of vertices describing a polygon:
> > 	a) Apply the delaunayn function in the geometry package to get a
> > Delaunay tessellation
> > 	b) Remove triangles that have any segments whose center points
> > are outside the polygon (using the point.in.polygon 
> function from the sp
> > package to identify these)
> > 	c) Remove triangles that have any segments that cross a line
> > segment comprising the polygon (using the crossing.psp 
> function from the
> > spatstat package to identify crossings)
> > 	d) Build new triangles for polygon segments that were orphaned
> > by steps b and c (to be done).
> >
> > This would be much simpler if there were a function to limit the
> > tessellation to the interior of the polygon in the first place. Does
> > anyone have or know of such a function? (Although there might be GIS
> > tools to do this, I am only interested in a solution in R 
> so that it can
> > be used in R scripts.)
> 
> Have you looked at the triangulation code in the gpclib package, even 
> though it is only 2D? Would it help pick out the internal triangles?
> 
> Roger
> 
> >
> > Brian Gregor, P.E.
> > Transportation Planning Analysis Unit
> > Oregon Department of Transportation
> > Brian.J.GREGOR at odot.state.or.us
> > (503) 986-4120
> >
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> > R-sig-Geo at stat.math.ethz.ch
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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
>