[postgis-users] Creating a boundary of an aggregate of buildings

[Tomas Straupis]

> unfortunately it does not work as expected, since I have always many
> many many little areas...
>
> Any other idea?

  Are you sure you're getting a combined multigeometry before applying
buffer+-? All small polygons which you expect to be aggregated into
one should be in one geometry (one row) before doing buffering.

  1. Increase buffer size.
  2. ST_ConvexHull, ST_ConcaveHull.

  Here is a working forest aggregation routine.

create table gen_forest as
  select nextval('gen_forest_seq') AS id
        ,0::bigint AS way_area
        ,10 AS res
        ,ST_CollectionExtract(unnest(ST_ClusterWithin(way, 10)),
3)::geometry(MultiPolygon, 3857) as way
    from planet_osm_polygon
   where landuse = 'forest';

delete from gen_forest where st_area(st_buffer(way, -10)) < 10 and res = 10;

update gen_forest set way =
st_makevalid(st_multi(st_simplifypreservetopology(st_buffer(st_buffer(way,
10, 'quad_segs=1'), -10, 'quad_segs=1'), 10))) where res = 10;
update gen_forest set way_area = st_area(way) where res = 10;

-- 
Tomas