[postgis-users] PostGIS topology use ?

strk strk at keybit.net
Thu Jul 1 09:16:26 PDT 2010

On Thu, Jul 01, 2010 at 09:43:33AM -0400, Stephen Woodbridge wrote:

> 1. working with polygon coverages to maintain boundary coherence during 
> edits
> 2. simplify adjacent polygons without leaving gaps between them.
> 3. editing road networks and maintaining coherence of joined segments 
> and intersections
> While these are more high level application ideas and I guess at the 
> lowest level you could build a topology that would support any of these, 
> where did not seem to be support for any higher level functions that 
> would help support implementing these ideas.

Right, not very management function reach at this time...

For (2) the simplification part it wouldn't be too hard to do.
Currently, if you use ST_Simplify(TopoGeometry) you'd get
an automatic cast from TopoGeometry to Geometry, and _then_
the Geometry would be simplified.
A simple override of that function could instead _first_
simplify components and _finally_ polygonize them togheter
to get you the Geometry. Would that meet your use case 2 ?

For (3) I don't think topology is for you. SQL/MM defined another
model which was "network". Networks have different characteristics
from topology. For example you may have intersections which do
not form nodes (think bridges).

Case (1) is the whole point of keeping the representation of
your data as a topology, but I'm not sure I understand what
you mean by 'during edits'. Postgis topology does have a
validation function you could run after edits, but I guess
your edits would be on the app level. For instance QGIS
itself does use some form of topological representation when
doing edits. Would be interesting to see it cope directly
with topology in the backend.


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