Try this: SELECT ST_Buffer(ST_Union(ST_Buffer(geom, 0.0001)), -0.00008) FROM points
> -----Original Message----- > From: [email protected] [mailto:postgis-users- > [email protected]] On Behalf Of Ivan Price > Sent: Tuesday, January 14, 2014 8:36 AM > To: PostGIS Users Discussion > Subject: Re: [postgis-users] ST_ConcaveHull giving up too soon, aka too > convex > > Thanks for your reply Remi, > > > > for now we are going with simply a union of a buffer of the points (as you > suggested) as it makes sense in the context of the origin of the data > anyway. > > > > cheers > > > > -i > > > > > > De : [email protected] [mailto:postgis-users- > [email protected]] De la part de Rémi Cura > Envoyé : Tuesday, 14 January 2014 11:39 > À : PostGIS Users Discussion > Objet : Re: [postgis-users] ST_ConcaveHull giving up too soon, aka too > convex > > > > Hey, > > sadly I can't answer your questions, > > but it looks like you don't want concave hull but instead Alpha Shape (see > SFCGAL , maybe they have integrated it). > > > You have still a lot of workaround : > if you don't mind expending the bouyndary, a union of buffers of points > if you don't mind quantization (anyway your input looks quantized) : > convert to raster, then any classical image processing easily available > (watershed for example) (you can use out of the box QGIS 2.0 raster > processing tools) > if you want a sharp boudary : and assuming your points are regularly > spaced : maybe something liek this (untested) > create a segment for each point to his K nearest neighbours (K would be 2 > or 3, and you would put a max distance before drawing a line) > > Then pick the segments that have no intersection with other segments, and > either use ST_Polygonyze, or cut the bounding box polygon with the > segments and remove the part outside > . > > Maybe you can try to reduce your point precision for processing (snap to > grid) and/or put the points in an appropriate srid (not too much digits) > > > > Cheers, > > Rémi-C > > > > 2014/1/14 Ivan Price <[email protected]> > > > > Hi there, > > > > I am experimenting with ST_ConcaveHull, am using Postgres 9.2.2 on > windows with postgis 2.1.1 r12113 > > > > i have a table of 218 points forming roughly an arc (something like the nike > swoosh) > > > > when i ask for the concave hull i'm expecting a poly shrink-wrapped to > them, however this is not the case, even when i set a very low percentage > area parameter. for example: > > > > SELECT ST_ConcaveHull(ST_Collect(geom), 0.000001) as geom FROM > points; > > > > i attach a screenshot of the points and the convext hull, as well as an sql > dump of the points themselves. > > > > also as a side note, in researching the problem i upgraded from postgis 2.01 > to 2.1.1 and noticed a change in the processing time for the above > statement from 1 minute to ~ 5 minutes. > > > > are there any pointers for making ConcaveHull more 'aggressive' ? > > > > thanks and regards > > > > -i > > > > > _______________________________________________ > postgis-users mailing list > [email protected] > http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users > > _______________________________________________ postgis-users mailing list [email protected] http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
