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

[Sandro Santilli]

On Fri, Sep 27, 2013 at 06:31:35PM +0200, Rémi Cura wrote:
> Sorry I wasn't clear,
> 
> I'm not saying ST_IsValid is wrong, I guess flat polygons are permitted (I
> don't know)

No, flat polygons (collapsed to line) are invalid.

> 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).

This would be a bug. Note that ST_Area may tell you 0 even if the real
area isn't zero but too small to be represented (tried to compare with 0 ?)

> 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

I know...

--strk;