[postgis-users] Simplify a perfect (hierarchical) partition of polygons (ST_SimplifyPreserveTopology and friends)

strk at keybit.net strk at keybit.net
Thu Mar 15 01:39:41 PDT 2012

I should probably correct myself: it may make sense to simplify single
TopoGeoms in turn if you do not want the "simplified topology" to
persist (but are happy with "just the simple geometries" output).

Only doing that (simplifying TopoGeom objects in a topologically-aware
way) is not currently implemented. You'd get the TopoGeom first cast
to a simple and then simplified, which would defeat the whole purpose.

See http://trac.osgeo.org/postgis/ticket/1687 if you want to start
contributing code :)


On Thu, Mar 15, 2012 at 09:07:08AM +0100, strk at keybit.net wrote:
> On Thu, Mar 15, 2012 at 12:45:43AM +0000, Martin Tomko wrote:
> > Hi Sandro, William, all,
> > Thank you very much for the responses.
> > 
> > Sandro, this looks hopeful. I have checked and indeed the
> > addTopologyColumn method contains the integer specifying a child layer. I
> > "assume" that this is taken from the topology.layer table where each
> > topological layer is registered, right?
> Right. It is also returned by AddTopoGeometryColumn.
> I know that function is not very friendly and I'm happy to discuss
> proposals to make it more so (like taking "schema.table.column" ?)
> > Sandro, you say:
> > Quote:
> > "Yes, it probably don't make sense to simplify single TopoGeoms in turn if
> > you know you're going to simplify the whole topology. In that case you
> > would
> > just make a copy of the topology and then simplify all edges on the copy.
> > Existing TopoGeometry objects (appropriately re-bound to the new topology)
> > would magically convert to a simplified version of simple geometries. Both
> > simple and hierarchical ones."
> > 
> > 
> > Is this a "wished for" functionality, or is this doable? I have little
> > experience with the topoGeom, a code snippet would make me very happy ;)
> It is doable. You can get code snippets to create TopoGeometry objects
> from the manual and presentations. For the "simplify whole topology"
> there's a bit more work involved but it starts at CopyTopology.
> > and we are an explicitly OS project. We are willing to contribute back
> > (also in due course to other projects, such as Geotools), but we cannot
> > contribute $, only code. For those interested, look up www.aurin.org.au
> Glad to know you're ready to contribute. Note that also analysis and
> discussion and testing are highly welcome. In this case I'm sure you
> could help defining an easy way to copy layers (there's a ticket on trac
> for a CopyTopologyLayer, I think). Only you'll need to understand 
> TopoGeometry model some more I think.
> --strk;
>   ,------o-. 
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