[postgis-users] Split polygons based on polygons' area

[Jibran Khan]

Hi,

Thanks a lot for your help. Here is my first attempt.

/*## Start ##*/
With      foo as(
Select
                gid poly_id, geom,
                ST_GeneratePoints(geom, 1000) ptcloud,
                (ST_Area(geom)/100)::int k
From
                My_polygon_shp
Group by gid, geom
Order by gid, geom),

                bar as(
Select
                poly_id, geom, ptcloud,
                ST_VoronoiPolygons(ST_Centroid(ST_Collect(ptcloud))) vorpoly,
                ST_ClusterKMeans(ptcloud, k) over() kmeans
From
                foo
Group by poly_id, geom, ptcloud, k
Order by poly_id)

Select
                poly_id,
                st_intersection(geom, vorpoly)
From
                bar
Order by poly_id
/*## End ##*/

The query may not be efficiently written but it works. However, I don’t get any significant result.
Please see if you can you reproduce the same error or above query can be improved?

Best,
Jb

From: postgis-users <postgis-users-bounces at lists.osgeo.org> On Behalf Of Darafei "Kom?pa" Praliaskouski
Sent: 17. juni 2018 12:26
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Subject: Re: [postgis-users] Split polygons based on polygons' area

The way I see it, for any kind of polygon:
 - Convert a polygon to a set of points proportional to the area by ST_GeneratePoints (the more points, the more beautiful it will be, guess 1000 is ok);
 - Decide how many parts you'd like to split into, (ST_Area(geom)/max_area), let it be K;
 - Take KMeans of the point cloud with K clusters;
 - For each cluster, take a ST_Centroid(ST_Collect(point));
 - Feed these centroids into ST_VoronoiPolygons, that will get you a mask for each part of polygon;
 - ST_Intersection of original polygon and each cell of Voronoi polygons will get you a good split of your polygon into K parts.

Will it work for you?

вс, 17 июн. 2018 г. в 13:11, Jibran Khan <jibran at envs.au.dk<mailto:jibran at envs.au.dk>>:
Hello everyone,

I need some help in terms of splitting polygons based on the polygon area. I am using PostgreSQL 9.5.12/PostGIS 2.3 on Windows 10 (x64). I have two tables i.e. polygon (poly) and points (pts) in my Postgres db. Some of the polygons contain points inside them while, some do not (sample scenario below):

[cid:image001.png at 01D40632.134FDF00]

The following query:

/*Start of query*/
Select
    a.gid poly_id, count(b.geom) pt_cnt,
    st_area(a.geom)::int poly_area
FROM
    poly a
LEFT JOIN
    pts b
ON st_contains(a.geom, b.geom)
GROUP BY a.gid
Order by a.gid;
/*End of query*/

Returns the area of polygons (from left to right) as 1079, 744, 340 square-meter, respectively. I need to split these polygons' geometry based on the area i.e. if area is above 100 meter-square, then each polygon needs to be divided/split into “approximate equal” parts (assuming that majority of the polygons are straightforward like above). The division does not have to be “strictly” equal. Does anyone know or have any idea how this can be achieved?

Side Notes:


•         I earlier posted this question here at GIS SE<https://gis.stackexchange.com/questions/286184/postgis-how-do-i-split-polygon-in-equal-parts-based-on-polygon-area>

•         Also, came across this<https://gis.stackexchange.com/questions/239801/how-can-i-split-a-polygon-into-two-equal-parts-along-a-n-s-axis> relevant question but I guess my problem is different

Any help or idea(s) would be highly appreciated.

Best,
/Jibran
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