[postgis-users] Topology and Triggers

[Dylan Beaudette]

On Thursday 15 May 2008, Bruce Rindahl wrote:
> Has anyone thought about the Arc/Info coverage model?  The idea would be
> to have a table with line geometries and attributes reflecting the left
> and right polygon ids.  My thought is you do your edits on the line
> geometry table and then polygon topology is built in a separate
> (dependent, inherited??) table based on the line table using well
> defined procedures.  The only function I  don't see offhand that is not
> there is a split polygon with line.  Just my thoughts.
>
> Bruce Rindahl

Along these lines (so to speak) the GRASS model for encoding topology would be 
another candidate.

Cheers,

Dylan


>
> Webb Sprague wrote:
> > On Thu, May 15, 2008 at 5:20 AM, George Silva <georger.silva at gmail.com> 
wrote:
> >> Hello all again,
> >>
> >> I´m curious. I´ve said in qgis list and some other 'desktop' GIS
> >> software lists that the biggest feature missing for OSGIS is something
> >> like the 'auto-complete' polygon in ArcGIS,, that would make life much
> >> easier.
> >
> > Indeed!
> >
> >> I´ve been reading some things, and i´ve seem people mentioning trigger
> >> based topology (eg cut out the overlaping area of digitized polygons,
> >> or,maintain only the overlaping areas of the digitized polygons, and
> >> other related issues).
> >
> > I think the hard part is deciding how you want to do this exactly,
> > since all approaches involve changing the data somehow; a lot of
> > people are chary of automatically deleting gaps, for example, lest
> > they aren't really gaps (consider the Vatican Republic in  a map of
> > euro nations...).
> >
> >> I imagine that would be a trigger using something like after insert
> >> update the_geom.
> >>
> >> Can you guys with more experience give me a example, so i can build my
> >> own? Best pratices? Some light around this subject?
> >
> > Have you ever written a non-geo trigger in postgresql?  If not, I
> > would practice  using the docs; I am pretty sure you could just modify
> > NEW with a geo operator/ function.
> >
> > If so, I think a larger scale project needs to be embarked on.  The
> > work so far seems to me to be quite low level without a grand plan,
> > and I think a grand plan is needed.  I would start with a bunch of
> > functions for modifying a geometry based on its neighbors and some
> > topology constraints, then maybe a schema to store topology
> > information, and only then start implementing triggers.  But this is
> > armchair quarterbacking--hopefully someone with some real experience
> > will say something...
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-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341