[postgis-users] Topological Exceptions

Chris Hermansen chris.hermansen at timberline.ca
Tue Mar 11 11:52:37 PDT 2008

Hi Regina and others;

This is an interesting suggestion.  Basically, as a design principle I
suppose one could argue that there's no point in throwing an exception
unless there's a way to catch it.  Just another way in which SQL lets us

I'm sure you're right; there appears to be no shortage of good people
out there with GPSes and ArcView making furry data for the rest of us :-)

I've had pretty good success with two approaches to getting rid of data
lint.  One is to use st_simplify() with a tolerance that's compatible
with my data resolution.  The other one that often works even better for
me is st_snaptogrid().

You could probably convince yourself that Nyquist's sampling theorem
would apply in this case, so that if your mapping process could not
distinguish data points closer than X meters apart, then you should use
X/2 as your tolerance for simplifying or snapping to a grid.

Obe, Regina wrote:
> I've been doing a lot of work using ST_Intersection, ST_Difference,
> ST_Union etc to cut out slices of geometries I don't want and slicing
> up geometries into smaller pieces, reunioning etc.
> I've been running into a lot of Topological Exceptions of the form
> directed Edge this and non-noded that.  For the most part I've been
> able to overcome these by slicing things smaller or skipping over
> problem regions (e.g. areas where the difference is a line or point or
> something of that sort).
> I think a lot of people have run into the same issues and have gotten
> frustrated and there doesn't seem to be a simple solution suggested to
> overcome these.
> Is there any way we can change (add overloaded functions or some
> setting parameter) - that take additional parameters to simply ignore
> these errors that says return empty collection or NULL when a GEOS
> error is thrown.  That way I can simply ignore these and throw them
> out of my equation and move on.
> Just a thought.  I'm sure my suggestion is rather simple-minded and
> I'm sure I'm missing some reason of why this is not feasible.  An
> explanation of why my suggestion is stupid at anyrate would be nice.
> Thanks,
> Regina
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Chris Hermansen · mailto:clh at timberline.ca
tel:+1.604.714.2878 · fax:+1.604.733.0631
Timberline Natural Resource Group · http://www.timberline.ca
401 · 958 West 8th Avenue · Vancouver BC · Canada · V5Z 1E5

C'est ma façon de parler.

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