Would the following work (if max_distance were available, which it is not in my distro)??:
SELECT ST_Buffer(centroid(the_geom), max_distance(centroid(the_geom), the_geom) as bounding_circle ; This assumes the following: 1. The Roeck test is essentially the "bounding circle" of a geometry 2. The centroid is the center of mass 3. the furtherest point from the center of mass will be enclosed by a circle of radius equal to the distance from said point to the center 4. The max_distance function would work with points and multi-polygons Thinking about a "bounding circle" makes me wonder: is this a useful concept for geometric indexing? Could this be used in conjunction with the bounding box to create a more refined, quick and dirty bounding box containment index? In other words, if using the && operator essentially throws out (or includes if NOT'ed) points outside a bounding box, would it be computationally efficient to do something like a comparison to a circle on the objects centroid? In essence, all you would have to query would be the centroid, and radius of the spheres, like (in pseudo-code): If (distance(c1, c2) <= (r1 + r2)) then circles_overlap = true; # where r1 and r2 are the radii of the two bounding circles, and c1 and c2 are their centroids With this simple comparison (maybe not so simple computationally, I dunno), we could cut out a good deal of the blank space in the "corners" of the bounding box. Anyone? Robert W. Burgholzer Surface Water Modeler Office of Water Supply and Planning Virginia Department of Environmental Quality [EMAIL PROTECTED] 804-698-4405 Open Source Modeling Tools: http://sourceforge.net/projects/npsource/ -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Lee Meilleur Sent: Friday, October 24, 2008 4:20 PM To: 'PostGIS Users Discussion' Subject: RE: [postgis-users] Roeck test My bad, you're both correct. If that's the case, what does the ST_ExteriorRing function produce? When I test it I get "geom is not a polygon". -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Paul Ramsey Sent: Friday, October 24, 2008 2:59 PM To: PostGIS Users Discussion Subject: Re: [postgis-users] Roeck test It's an inscribed circle? I thought it was a containing one. P. On Fri, Oct 24, 2008 at 12:50 PM, Lee Meilleur <[EMAIL PROTECTED]> wrote: > > > The area of the convex hull is actually larger than the area of the > polygon, so the ratio ends up being greater than 1. With the Roeck > test, the ratio will always be between 0 and 1, with a compact polygon > approaching 1 (the area of the inner circle approaching the area of > the polygon). A ratio approaching 0 would indicate a gerrymandered > district. > > Assuming my statement is correct that is: > > SELECT congress.district, area(ST_ConvexHull(congress.the_geom)) / > area(congress.the_geom) AS convex_roeck FROM congress; > > Bruce, thanks for the idea, I'll see what I can produce from my > minimal experience with stored procedures. > > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Martin Davis > Sent: Friday, October 24, 2008 11:40 AM > To: PostGIS Users Discussion > Subject: Re: [postgis-users] Roeck test > > > Wouldn't it be easier and more accurate to use the convex hull of the > polygon instead of the minimum enclosing ball? The CH will have > smaller area (i.e. be a tighter bound on the area covered by the > polygon), so it seems like it would give a more accurate basis for > comparison. > > Lee Meilleur wrote: >> Has anyone calculated the Roeck test using PostGIS? This is an area >> based measure that computes the ratio of the area of a polygon to the >> area of the minimum enclosing circle for the polygon. It's used to >> measure compactness of districts for redistricting applications, >> basically a way to test for gerrymandering. Thanks. >> >> Lee Meilleur >> LCC-GIS Office >> Minnesota Legislature >> 100 Rev. Dr. MLK Jr Blvd. >> 55 State Office Building >> St. Paul, MN 55155 >> 651.296.0098 Fax: 651.297.3697 >> [EMAIL PROTECTED] >> http://www.gis.leg.mn >> >> >> _______________________________________________ >> postgis-users mailing list [email protected] >> http://postgis.refractions.net/mailman/listinfo/postgis-users >> >> > > -- > Martin Davis > Senior Technical Architect > Refractions Research, Inc. > (250) 383-3022 > > _______________________________________________ > postgis-users mailing list [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users > _______________________________________________ > postgis-users mailing list > [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
