[postgis-users] Possible topology bug

[Sandro Santilli]

On Tue, Apr 16, 2013 at 05:05:09PM -0700, pcreso at pcreso.com wrote:
> Hi,
> 
> I'm trying to determine all the possible intersections in sets of polygons.
> 
> The approach I'm using is to load each set of polygon perimeters into a Postgis topology & extract all the faces as geometries. This results in some new polygons lying between the input ones. Which is the nature of this technique- that is not the problem.
> 
> I then determine the number of original polygons that the new ones overlap, which should go from 0 up. My problem arises in that I'm getting new polygons, derived from the boundaries of the old ones, which are actually between old polygons, but have a non-zero (but very small - like 1e-15) area of overlap - but all they have in common is the boundary line which should have a zero area.
> 
> So some of my new polygons supposedly overlap an old polygon, but in fact they do (or should) not. 

How are you checking the overlap area ?
And how are you checking what polygons have "in common" ?

The thing is that building the topology implies noding the input
which in turn can change its shape. As a result the final edges are not
always fully covered by the original linework.

--strk;