Archive for March, 2013

On the fly simplification of topologically defined geometries

Friday, March 8th, 2013

Keeping GIS data in full-resolution and simplifying it on demand is a known challenge: simplification has to be fast and its output has to be topologically consistent.

simplified geometriessimplified topogeometries

We saw how to get a topologically consistent simplified version of a full layer, but that method isn’t fast enough for on-demand usage. Also, we saw how to perform a fast simplification by sacrificing the degree or generalization so that the introduced inconsistency would not be visible on a rendering surface.

An approach balancing speed and quality would take advantage of the topological definition of geometries to constraint the simplification work by ensuring shared edges get an identical treatment.

This is now possible with the addition in PostGIS of a  new version of ST_Simplify accepting a TopoGeometry as input.

In the picture above you can see the difference between using Geometry or TopoGeometry objects for simplifying the provinces of Tuscany with a tolerance of 8km. The ST_Simplify function runs at comparable speed in both cases.