# On the fly simplification of topologically defined geometries

Posted on by strk

Category: Free Software GIS

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**.

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.