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 >
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