[postgis-users] Creating a simple polygon encompassing a given number of points

[pcreso at pcreso.com]

Hi Rene,

Perhaps something like this?

select region, ST_simplify(ST_buffer(ST_convexhull(ST_collect(geom), n),n/3)) from table group by region;


ST_simplify removes extraneous vertices - increase the n/3 parameter to remove more vertices, but you'll also need to increase n to ensure all the points lie within the simplified polygon - you don't have to use n/3, just a value off the top of my head

ST_buffer will create a new polygon distance n outside the original one

ST_convexhull will create a polygon encompassing the points. ST_concavehull might also work, but I've found it problematic.

ST_collect will create a group of points to work with

all grouped by region.



HTH,

   Brent Wood


--- On Tue, 9/11/12, René Fournier <m5 at renefournier.com> wrote:

From: René Fournier <m5 at renefournier.com>
Subject: [postgis-users] Creating a simple polygon encompassing a given number of points
To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
Date: Tuesday, September 11, 2012, 6:39 AM

I've written a script to migrate location data from MySQL to PostGIS.
Each region contains 5-20 locations. I would like my script to create a simple polygon for each region that encompasses its locations (lat/lng points). I say simple in the sense of not too many vertices (< 10), even if the region contains 20 points.(The easiest thing would be a bounding box, but I would like it a bit more defined than that.)  Additionally, I would like the polygon to be a bit bigger (say, by 10 meters) than the points it contains. 

Just wondering if this is something that can be done in a PostGIS query or PHP using some algorithm for simple. What would you recommend? Thanks!

...Rene
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