[postgis-users] ST_ConcaveHull giving up too soon, aka too convex

[Ivan Price]

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

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