Strategies for grouping polygons

Question

I'm looking for strategies to replicate the functionality available via ArcGIS' "Aggregate Polygons" tool. This is different from a straight dissolve on the polygons - the operation has to fill in gaps within a certain tolerance.

I'd be happy with a solution in either QGIS or PostGIS. I tried to work something out already with a convex hull but it doesn't quite get what I'm after so I'm at a loss. Any strategies you've come across?

Answer 1

You can try something using either ST_ConvexHull or ST_ConcaveHull. Obviously, this is just a quick hack, but it might work in your circumstances.

Here's my original polygons:

Here's a union of the pairwise convex hulls for polys <= 500m apart:

select st_union(geom) as geom 
from (
  select st_convexhull(st_collect(a.geom, b.geom)) as geom 
  from polys a, polys b
  where st_dwithin(a.geom, b.geom, 500) and a.gid<b.gid
) t;

Here's a union of the pairwise concave hulls with 0.5 for the argument:

select st_union(geom) as geom 
from (
  select st_concavehull(st_collect(a.geom, b.geom), 0.5) as geom 
  from polys a, polys b 
  where st_dwithin(a.geom, b.geom, 500) and a.gid<b.gid
) t;

Things get weird if you enable holes in ST_ConcaveHull: