[postgis-users] PostGIS topology use ?

Stephen Woodbridge woodbri at swoodbridge.com
Thu Jul 1 08:31:20 PDT 2010

Andrea Peri wrote:
>>1. working with polygon coverages to maintain boundary coherence during 
>>2. simplify adjacent polygons without leaving gaps between them.
>>3. editing road networks and maintaining coherence of joined segments 
>>and intersections
> I'm very interesting to understand how actually the topology package act 
> on gaps between polygons.
> I think a topology environment must fill a gap with a polygon (with 
> empty attributes) instead of a simplify of polygons to remove the gap.

The idea behind topology is the if you have two polygons with a common 
edge, the the common edge is only represented as "one edge" and both 
polygons share that edge. if you make changes to that edge, then the 
polygons both have the same edge so it is not possible to create a gap 
between the two polygons.

In normal geometry each polygon has its own copy of the edge. So if you 
change one polygon there is not way to enforce changes on the adjacent 

In topology you have points. lines/edges/rings are made up of an order 
list of references to points. polygons are made up of an ordered list og 

Since lines and polygons all share references to points, if you move a 
point in an edit operation then all the geometry that references that 
point is updated and topological relationships remain in tact. You still 
need to do some geometric testing to check for validity of a point move, 
but that could be done in a set of move rules.


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