Hi Chris, Sorry, I didn't put that well, I meant the centroid of the extent (bbox) is defined by the outliers, not the centroid of the dataset, in my hierarchy of arbitrary representativeness :-)
I ain't really a PL/R user, but I'd like to be one. Only 24 hrs in the day, & I don't know R well enough, I have several workmates who are more than competent with R, & I work with them for R expertise - much easier & there are already too many things I don't do well!. Back on topic, I'd regard a centroid as the ultimate endpoint of a clustering analysis, & while in PostGIS it does seem to be a simple averaging approach, there are many more ways to do this. See http://en.wikipedia.org/wiki/Cluster_analysis for some examples of how you might otherwise determine a centroid. Or look up weighted averaging... Cheers, Brent --- On Wed, 9/16/09, Chris Hermansen <[email protected]> wrote: > From: Chris Hermansen <[email protected]> > Subject: Re: [postgis-users] Center of Points Collection > To: "PostGIS Users Discussion" <[email protected]> > Date: Wednesday, September 16, 2009, 7:15 AM > Cool, a PL/R user! > > Brent, I'm curious about why you make a distinction between > the centroid and the average, because as far as I can tell > they are the same thing. > > [email protected] > wrote: > > Hi guys, > > > > A bit more difficult, & way out in left field, but > if you use PL/R to create a median function for Postgres, > you could build your point from the median(X) & (Y) > values instead of the average. > > > > Where this would actually lie obviously depends on the > distribution of the points. The centroid is most affected by > (actually defined by) outlying points, the avg somewhat less > & the median less still. > > > > Of course once you have PL/R to play with, you have > much more flexibility to look at returning statistics from > datasets than just the median. > > > > Cheers, > > > > Brent Wood > > > > > > > > > > --- On Tue, 9/15/09, Kevin Neufeld <[email protected]> > wrote: > > > > > >> From: Kevin Neufeld <[email protected]> > >> Subject: Re: [postgis-users] Center of Points > Collection > >> To: "PostGIS Users Discussion" <[email protected]> > >> Date: Tuesday, September 15, 2009, 8:51 AM > >> Paul Ramsey wrote: > >> > >>> Faster than creating a multipoint is to > recognize that > >>> > >> ST_Centroid() > >> > >>> is just going to return the center of the bbox > of the > >>> > >> collection > >> > >>> anyways... > >>> > >> Unfortunately, Paul, ST_Centroid returns the > center of > >> mass, not the center of the bbox. > >> > >> SELECT astext(st_centroid(st_collect(column1))), > >> FROM (values ('POINT(0 0)'), > >> > ('POINT(0 1)'), > >> > ('POINT(0 2)'), > >> > ('POINT(1 0)')) as foo; > >> astext > >> ------------------ > >> POINT(0.25 0.75) > >> (1 row) > >> > >> Your second post, taking the avg of the x,y does > seem to be > >> the nice approach, and produces the same results > as > >> ST_Centroid - the center of mass. > >> > >> SELECT astext(st_makepoint(avg(st_x(column1)), > >> avg(st_y(column1)))) > >> FROM (values ('POINT(0 0)'), > >> > ('POINT(0 1)'), > >> > ('POINT(0 2)'), > >> > ('POINT(1 0)')) as foo; > >> astext > >> ------------------ > >> POINT(0.25 0.75) > >> (1 row) > >> > >> If Dustin is after the center of the collection, > then > >> something along your first suggestion might be > more > >> appropriate. > >> (taking the center of the extents) > >> > >> Cheers, > >> Kevin > >> _______________________________________________ > >> 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 > > > > > -- Regards, (please note new mobile number below) > > Chris Hermansen > mailto:[email protected] > tel+1.604.714.2878 · fax+1.604.733.0631 · > mob+1.778.840.4625 > Timberline Natural Resource Group · http://www.timberline.ca > 401 · 958 West 8th Avenue · Vancouver BC · Canada > · V5Z 1E5 > > _______________________________________________ > 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
