R-sig-Geo Digest, Vol 43, Issue 14
Hi Nic, The convex hull would be fast and easy to compute (there's existing code in R). I want the ordinary hull which is the set of arcs forming the perimeters (inside and out). My crude and very slow solution was to convert all the polygons (in this case hexagons on a lattice) into their constituent arcs and then for each arc count how many times it occurs in the set (requires slightly fuzzy matching of points). Arcs that occur more than once are removed. The remaining arcs form the hull. Runs in about 20 minutes with a few hundred hexagons. Sufficient for the moment. THK
On 3/16/07, Nicholas Lewin-Koh <nikko at hailmail.net> wrote:
Hi Tim, I am not quite sure what you are getting at here. Do you want to intersect polygons and then select the set of lines that form the outer perimeter? Do you wan the convex hull of a set of polygons. I guess I have been out of the GIS world to long. It seems to me that this would be something easy to solve, just tedious iteration of the polygon coordinates and some triangulation. Nicholas
Date: Thu, 15 Mar 2007 10:49:23 -0500
From: "Tim Keitt" <tkeitt at gmail.com>
Subject: [R-sig-Geo] polygons to arcs?
To: r-sig-geo at stat.math.ethz.ch
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<6262c54c0703150849qe60ab14nfef1eb3bf73dfb5d at mail.gmail.com>
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Is there an 'sp' function that takes a polygon as its argument and
returns a set of line objects corresponding to the arcs in the
polygon?
Or better yet, a function that given a set of polygons, returns the
hull? (ie the set of singleton arcs after applying the polys to arcs
function)
THK
--
Timothy H. Keitt, University of Texas at Austin
Contact info and schedule at http://www.keittlab.org/tkeitt/
Reprints at http://www.keittlab.org/tkeitt/papers/
ODF attachment? See http://www.openoffice.org/
Timothy H. Keitt, University of Texas at Austin Contact info and schedule at http://www.keittlab.org/tkeitt/ Reprints at http://www.keittlab.org/tkeitt/papers/ ODF attachment? See http://www.openoffice.org/