agenda for R's Geo capabilities?
The thread below and many others in R-sig-Geo raise questions about
future directions. In reinventing GIS there are a whole list of
capabilities and functions that would be helpful. Some that I have
noticed include,
Topological representation to enable
Planar enforcement of boundary integrity of polygon tessellations
"Dissolving" interior edges easily as in the thread below
Large problem computational geometry functions
Identify many points inside of many polygons
Intersections/overlays of two sets of many polygons
Distances between all pairs of many polygons
Are there members of the R Geo community working on any of these?
Are these issues seen as an exclusive focus of commercial GIS?
Are there discussions about these issues at relevant conferences?
(I will be at AAG in San Francisco and would be happy to meet with
others if there is interest.)
r-sig-geo-bounces at stat.math.ethz.ch wrote on 2007-03-16 07:52:59:
Hi Tim, You could compute the convex hull first, and then iterate from points on the convex hull. That should be much faster already, especially
since
hexagons are convex and the perimeter will be locally convex around
all
the points touching the convex hull. You could do a variation of the "monotone pieces" algorithm that is used in computational geometry. But this is a simpler problem. Are there cases with interior holes? I have been meaning to write something like this for hexbin for a
while.
There are many cases where it would be nice to find approximations to the density contours and a quick and dirty way is to threshold the hexagon counts, find the hull and smooth the perimeter. Nicholas On Fri, 16 Mar 2007 08:34:20 -0500, "Tim Keitt" <tkeitt at gmail.com>
said:
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 Message-ID:
<6262c54c0703150849qe60ab14nfef1eb3bf73dfb5d at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed 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/
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