[postgis-users] Roeck test

[Lee Meilleur]

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: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] 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 <lee.meilleur at gis.leg.mn> 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: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] 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
>> lee.meilleur at gis.leg.mn
>> http://www.gis.leg.mn
>>
>>
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>>
>
> --
> Martin Davis
> Senior Technical Architect
> Refractions Research, Inc.
> (250) 383-3022
>
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