[postgis-users] Overlapping of polygons with SimplifyPreserveTopology

[Sandro Santilli]

On Wed, Dec 11, 2013 at 11:41:06AM +1100, Luca Morandini wrote:
> On 10/12/13 17:00, Paolo Cavallini wrote:
> >
> >Hi Luca,
> >you really need to use full topology for this.
> >ST_SimplifyPreserveTopology simply prevents topological errors, it
> >does not guarantee sharing of nodes etc.
> 
> Thanks for your answer Paolo, but our issue is how prevent
> simplification when it yields overlapping polygons: could a
> "universe polygon" solve it ?

A topological approach would solve it.
Simplifying a polygon may overlap with the still-unsimplified
adjacent one, so you can't check after each single simplification.
You could check afterwards, at the end, but then resolving one
overlap on one side may introduce another overlap in the other side.

Really, you need to model your data topologically, then simplify
edges, rather than full polygon boundaries. 
See http://strk.keybit.net/blog/2012/04/13/simplifying-a-map-layer-using-postgis-topology/
or http://strk.keybit.net/blog/2013/03/08/on-the-fly-simplification-of-topologically-defined-geometries/

--strk;