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

[Martin Davis]

You might also check out this paper:

http://www.foss4g.org/index.php/foss4g/2008/paper/view/226/157

It uses a "connect-buffer-simplify" approach. 

Martin Davis wrote:
> 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