[postgis-users] Topology and Triggers

George R. C. Silva georger.silva at gmail.com
Thu May 15 11:24:33 PDT 2008

I will try a geo-trigger now (that im at office).

But there are lots of complications that i can imagine...

I would like to hear from others if they have some experience in this. 
At this moment i will try a Topology trigger that will delete the 
overlaping part of digitized polygon.

Thanks for your input Webb!


Webb Sprague escreveu:
> 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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