Thank you! On Fri, May 27, 2011 at 9:33 PM, Stephen Woodbridge <wood...@swoodbridge.com > wrote:
> Aren, > > Your purposed approach sounds reasonable to me. You can do it all in one > query like: > > select c.gid, sum(c.dist*c.dist)/count(*) as variance > from ( > select b.gid, b.cent, st_distance(b.geom, b.cent) as dist > from ( > select a.gid, (st_dump(a.the_geom)).geom as geom, > centroid(a.the_geom) as cent > from ( > select 99 as gid, 'MULTIPOINT(1 2,2 3,3 4,4 5)'::geometry as > the_geom > union all > select 88 as gid, 'MULTIPOINT(1 2,2 3,3 4,4 5,3 5,9 > 9)'::geometry as the_geom > ) as a > ) as b > ) as c > group by gid order by variance desc; > > You should be able to replace the select...union all select ... with your > table of multipoints. > > -Steve W > > > On 5/27/2011 6:19 PM, Aren Cambre wrote: > >> Did anyone have thoughts on this? :-) >> >> Aren >> >> On Wed, May 4, 2011 at 2:12 PM, Aren Cambre <a...@arencambre.com >> <mailto:a...@arencambre.com>> wrote: >> >> The more I think about it, is this a job for R? I know I need to >> start using R at some point, just haven't begun yet. >> >> Aren >> >> >> On Wed, May 4, 2011 at 1:42 PM, Aren Cambre <a...@arencambre.com >> <mailto:a...@arencambre.com>> wrote: >> >> Suppose you have a geometry type with a multipoint. How would >> you calculate the variance of the points in that multipoint? >> >> I looked through the PostGIS 1.5 function reference and am not >> coming up with any easy way. >> >> A hard way seems to be using st_centroid(multipoint) to find the >> multipoint's center. From there, I can calculate the distance >> of each point from its center, and use that towards calculating >> the variance (each distance is squared, all squared distances >> are added together, then divide by number of points). >> >> I guess my ultimate need is to measure relative dispersion of >> multipoints. The multipoints that have the most dispersion are >> suspect, but I need a way of identifying which ones are like this. >> >> Aren >> >> >> >> >> >> _______________________________________________ >> postgis-users mailing list >> postgis-users@postgis.refractions.net >> http://postgis.refractions.net/mailman/listinfo/postgis-users >> > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users >
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