Theissen polygons

[julied at robinhood.ssmc.noaa.gov]

I kept track of the Theissen polygon discussion (Sept 95) because I have a
need to make Theiseen polygons for a project I'm working on.
I tried to use s.surf.idw with the # of interpolation points set to 1, but 
s.surf.idw takes into consideration distance and value of cells.
The theissen polygons I need can't take the value of the sites, I only
want the distance to be weighted. I haven't gotten s.geom to work, will
it solve my problem?


Here is the discussion:

=============================================================================
From: stewalt at ncrds.er.usgs.gov (Sue Tewalt x6437)
Subject: creation of polygons from points
Date: Wed, 27 Sep 1995 12:05:53 -0400 (EDT)

Someone posed a question to me about delineating polygons around
points (sites), sort of splitting the distance between points to create
a "polygon of influence" of each site. The sites are unequally distributed.

Would this best be done by converting the sites to a raster and using
r.neighbors or an r.mapcalc statement to "grow" the site cells?

Advice would be appreciated--- thanks in advance

stewalt at ncrds.er.usgs.gov
++++++++
Date: Wed, 27 Sep 1995 13:27:53 -0400
From: cliff at bellcore.com (Clifford A Behrens)
Subject: Re: creation of polygons from points

This sounds like a standard Voronoi tessellation problem to me.  There is a
tar distribution called s.geom in /incoming on the moon site that contains
modules to generate these polygons with GRASS.

Cliff Behrens
++++++++
Date: Wed, 27 Sep 1995 15:39:18 -0400 (EDT)
From: Angus Carr <apcarr at FLASH.LakeheadU.CA>
Subject: Re: creation of polygons from points

It sounds like you want to do Thiessen polygons.
I know they can be done with Arc/Info, but I'm new to GRASS.
++++++++
From: charnotw <charnotw at helios.aston.ac.uk>
Date: Tue, 3 Oct 1995 09:57:29 +0100
Subject: Re: creation of polygons from points

I haven't been following this thread closely so someone may have
mentioned this but you can make veronio/theisson/proximal polygons
by using s.surf.idw and setting the number of interpolation points
to 1.  You get a raster rather than a vector map but it's quick.

cheers  Tom  8-)
++++++++
Date: Wed, 27 Sep 95 12:22 CDT
From: James Darrell McCauley <mccauley at mcs.com>
Subject: Re: creation of polygons from points

I believe that the term that you are looking for is "voronoi diagram."
This may be found using the GEOM package:
  ftp://moon.cecer.army.mil/grass/incoming/geom.tar.Z

Darrell

P.S. avoid temptation and do not use s.voronoi in src.contrib.
     It has a bug which I do not care to fix (GEOM is much
     more extensive).
James Darrell McCauley, PhD           http://www.mcs.com/~mccauley/
16W518 Honeysuckle Rose Lane #214     mccauley at mcs.com
Hinsdale, Illinois 60521 USA          tel: 708.655.0643
+++++++
Date: Wed, 27 Sep 95 11:30:13 PDT
From: alhinc at cicese.mx (Alejandro Hinojosa Corona)
Subject: Re: creation of polygons from points

You could try v.cicle, here is a piece of the man page:

     v.circle - Creates one or more "polygon" circles in a binary
     vector file.  These circles are created using each point
     from an existing "site_lists" file as the centers for those
     circles.  There is one circle created per "site_lists"
     point.
     (GRASS Vector Program)

Alejandro Hinojosa
=============================================================================


I hope this helps.


Julie Daniel
julied at littlejohn.ssmc.noaa.gov