[postgis-users] Get concave hull from an array of longitude in latitude points

[Martin Davis]

I think the ideas for the solution have already been posted on this 
thread - you either need to use alpha shapes or one of the concave hull 
algorithms.  It sounded like pgrouting has an alpha shapes 
implementation - that would probably be the easiest way forward.  
Implementing the other algorithms would be non-trivial.

It might also help if you posted a picture of what you think the desired 
output should be, for the sample data you posted.

Aleš Kecman wrote:
> Hi!
>
> I'm still struggling with this "concave hulls" and it seems I just 
> can't find the way how to do it. I'm sending a text file with some 
> longitude and latitude points of selected subarea from which I need to 
> get a concave polygon. There is only one polygon in attached text 
> file. In my database I have around 500 000 points from which I need to 
> form around 700 polygons. If there is a way to do it in PostGis I 
> would really appreciate any help. I also attached a jpg picture of my 
> convexhull polygon. Thanks!!!
>
> Greets, Ales.
>
>
>
> On Mon, Nov 24, 2008 at 12:13 PM, Mark Cave-Ayland 
> <mark.cave-ayland at siriusit.co.uk 
> <mailto:mark.cave-ayland at siriusit.co.uk>> wrote:
>
>     Aleš Kecman wrote:
>
>         Hi!
>
>
>         I'm new to Postgis and would really like to get some help on
>         getting a concave hull from an array of points.
>
>         For example:
>
>         I have an array of points - longitude and latitude of
>         buildings in particular area in the city. I already managed to
>         get a convex hull polygon from this array of points, but this
>         is not really helpful. What I'm trying to get is some kind of
>         concave hull polygon from an array of points, which I would
>         further like to use in Google Earth kml file.
>
>         Thanks.
>
>
>
>     Hi Aleš,
>
>     Does ST_Union() help you at all?
>
>
>     ATB,
>
>     Mark.
>
>     -- 
>     Mark Cave-Ayland
>     Sirius Corporation - The Open Source Experts
>     http://www.siriusit.co.uk
>     T: +44 870 608 0063
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022