[postgis-devel] (Bug?) Strange Behavior with ST_SymDifference (line, surface)

[Rémi Cura]

Sorry I wasn't clear,

I'm not saying ST_IsValid is wrong, I guess flat polygons are permitted (I
don't know)
I was just pointing out that valid geometry for Postgis may be invalid for
QGIS.

And St_SymDiff creates polygons instead of lines (I mean a polygon with 0
area instead of a linestring).
Snapping have solved the problem for me as this 0 area polygon is not
created at all.

However even with snapping I have numerical issues
Cheers
Rémi-C

2013/9/27 Sandro Santilli <strk at keybit.net>

> On Fri, Sep 27, 2013 at 06:11:53PM +0200, Rémi Cura wrote:
> > another food for thought :
> > ST_SymDifference produces polygon with no area (ie lines), and polygon
> with
> > self intersecting boundary.
> >
> > First image shows result of a buffer and SYmDifference based processing
> > Second image : I moved manually things to expose errors, I added no
> points.
> >
> > It surely is a precision issue I'll try to solve with snapping.
> >
> > Yet all the geometries passes ST_IsValid, but are invalid for QGIS
>
> Are you saying ST_IsValid is wrong ?
> If that's the case please provide HEXWKB of the geometry you think
> is invalid. Best if in a ticket.
>
> A line from overlay involving areas is an expected possibility.
>
> --strk;
>
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