[postgis-users] Use of tolerances in topology

Sandro Santilli strk at keybit.net
Tue Apr 17 08:46:07 PDT 2012

On Tue, Apr 17, 2012 at 05:35:42PM +0200, Jose Carlos Martinez wrote:
> Hi Sandro, Im learning from topology.sql how the topology functions
> are working with more detail.
> I have a question about how the tolerances are managed, for example:
> I see (ST_AddIsoEdge) that you have used some times spatial
> predicates as st_contains or st_intersects. As you know these
> predicates (and others too) just work in a good way if the
> geometries are snapped to each other previously,  so my question is
> why you didnt use st_Dwithin with a tolerance? or maybe these
> functions expect the geometries to be already snapped by totopogeom
> for example and they shouldnt be used directly?

Those functions are ISO functions and as such are not specified to
deal with a tolerance. Handling of tolerance is not always
straightforward as using ST_DWithin. It may also have a big cost.

So take all those functions as being expected to be already snapped.

I'm open to suggestion about enhanced versions taking tolerance into
consideration. But it must be possible to invoke such functions in
a fast way, when caller already did their checking and noding and what

The idea is that all the ST_ prefixed functions end up being just wrappers
around more flexible functions in core.


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