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
