[postgis-users] Use of tolerances in topology

Paul Ramsey pramsey at opengeo.org
Tue Apr 17 09:58:01 PDT 2012

Your understanding is correct, the approach is mostly brute force, and
I have a pile of code lying about waiting to go in that implements
what you describe in terms of building internal indexes.


On Tue, Apr 17, 2012 at 9:29 AM, Jose Carlos Martinez
<jomarlla at cgf.upv.es> wrote:
> Thanks Sandro, I understand now.
> About  trying to enhance ST_Dwithin, last time I checked PostGIS code it was
> implemented using brute force (after comparing the boxes). Maybe one way to
> improve the algorithm (DT_Dwithin) will be to use spatial indexes inside a
> geometry. According to my tests (long time ago with JTS) I remember it could
> be faster to build a spatial index every time for a geometry and use it than
> using brute force (with geometries with more than 20 vertexes or so).
> Anyways Im not sure if PostGIS is still using brute force with st_dwithin,
> if so then forget everything I said.
> Regards,
> On 17/04/2012 17:46, Sandro Santilli wrote:
>> 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
>> not.
>> The idea is that all the ST_ prefixed functions end up being just wrappers
>> around more flexible functions in core.
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