[postgis-devel] topology expectations?

[Sandro Santilli]

On Wed, Mar 21, 2012 at 09:44:00AM +0100, Sandro Santilli wrote:
> On Tue, Mar 20, 2012 at 03:55:34PM -0700, Paul Ramsey wrote:

> > cded=# select TopoGeo_AddPolygon('sheds', st_geometryn(geom,1), 5.0)
> > from sheds where gis_tag like 'VICT%';
> > 
> > ERROR:  Spatial exception - geometry intersects edge 39
> > CONTEXT:  PL/pgSQL function "topogeo_addlinestring" line 175 at assignment
> 
> Tolerance uses ST_Snap which in turn is known to possibly introduce geometry
> invalidities [1]. Not always snapping to the _closest_ item may also
> introduce unpredictable outcomes.
> 
> [1] http://trac.osgeo.org/geos/ticket/501

For the record: the topology construction robustness issue was fixed:
http://trac.osgeo.org/postgis/ticket/1706

The problem was confirmed being about use of ST_Snap for handling tolerance,
that is: specifying a tolerance could currently do worst than not
specifying one, due to ST_Snap doing a suboptimal work.

If anyone wants to help with improving ST_Snap please do with ideas here,
on the wiki [2], in the GEOS ticket [3].

[2] http://trac.osgeo.org/postgis/wiki/ToleranceDiscussion
[3] http://trac.osgeo.org/geos/ticket/501

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