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:[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]<mailto:[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]<mailto:[email protected]> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
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