On Wednesday 09 January 2008, David William Bitner wrote: > ST_pointonsurface() will return a point that is guaranteed to be on the > polygon. I believe that this point is the point that is at a maximum > distance from any edge. This may be more useful for your case than a > centroid. There is no such thing as a logically correct "centroid", there > is only what a centroid *is* and what it is may not be on the surface of a > polygon.
David, Thanks for the clarification. I suppose that I should have looked over the documentation a little closer. The ST_pointonsurface() function does exactly what I needed. Apologies for attempting to redefine the meaning of "centroid" for my own nefarious purposes [1]. Cheers, Dylan 1. "Fun" way to interact with USDA-NCSS soil survey data in Google Earth (http://casoilresource.lawr.ucdavis.edu/soil_web/kml/mapunits.kml). Powered by PostGIS of course. > On Jan 9, 2008 3:45 PM, Dylan Beaudette <[EMAIL PROTECTED]> wrote: > > Hi, > > > > I have noticed that the ST_centroid() function will compute > > _geometrically_ > > correct centroids, but will not always compute _logically_ correct > > centroids. > > > > For example, given two polygons defining hillslope position such that one > > polygon defines the "ridge" and another defines the "slope". These two > > polygons are roughly concentric and therefore the geometric centroid of > > the > > outermost polygon is computed to exist somewhere in the innermost > > polygon. > > > > Are there any known ways to make postgis return a logically correct > > centroid: > > i.e. in the presence of concentric polygons force the position the > > centroid > > to be within the correct polygon. > > > > Thanks! > > > > Dylan > > > > -- > > Dylan Beaudette > > Soil Resource Laboratory > > http://casoilresource.lawr.ucdavis.edu/ > > University of California at Davis > > 530.754.7341 > > _______________________________________________ > > postgis-users mailing list > > [email protected] > > http://postgis.refractions.net/mailman/listinfo/postgis-users -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
