[postgis-users] Method to remove overlaps in a layer

Sandro Santilli strk at keybit.net
Mon Mar 19 13:47:22 PDT 2012

On Mon, Mar 19, 2012 at 02:16:30PM -0400, Pierre Racine wrote:
> > You should really be constructing a topology out of that dataset, you know ?
> > Will be probably slow but I'm sure you can survive :)
> So what would be the normal/easiest steps to convert a messy polygon coverage into a clean topology? Is it documented somewhere?
> My guess:
> 1- SET search_path TO topology,public;

You shouldn't need this, when you load topology.sql you should
get "topology" already appended to the end of the search_path
associated with the database.

> 2- SELECT CreateTopoGeom('test')


> 3- SELECT toTopoGeom(geom, 'test', 1)
> FROM mymessyone;

You didn't create a layer, see AddTopoGeometryColumn.
The third argument is a layer id, as returned by that function.

> or
> SELECT ST_CreateTopoGeo('test', geom)
> FROM mymessyone;

This one only works starting with an empty topology so
you'll need to pass it a full collection:

SELECT ST_CreateTopoGeo('test', ST_Collect(geom)) FROM mymessyone;

But I'd recommend using toTopoGeom instead, to keep the linking
between attributes and geometries.

> What happens when a polygon to be added to the topology overlaps a polygon already in the topology?

Two overlapping rectangles would produce a total of 3 faces.
If you're returning TopoGeometry objects (your step 2) both
of them will be defined by 2 faces, having one face in common.


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