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
