[postgis-users] Polygon validity

[Martin Davis]

Andrea Peri 2007 wrote:
> I think it would be invalid only in the domain of simple-features.
>
> Not in general.
>
> So I think is right think that for a geometric linestring 
> selfintersect, or having some consecutive or not consecutive (but 
> always internal) point repeated is invalid for a simple-feature world,
> but it can be valid for the more huge world of "not simple-feature" 
> geometries.
>
>> Another thing to consider - if you change the semantics of isSimple 
>> to report false for linestrings containing duplicate points, then you 
>> have no way of telling the difference between linestrings which 
>> contain true, topological self-intersections and ones which just 
>> happen to contain a topologically irrelevant duplicate point.
>
> Why it is irrilevant ?
>
See my previous post. 

"This is why I say that repeated points are topologically irrelevant - 
they are topologically indistinguishable under any continuous 
parameterization function."

I think there's some confusion here between the textual and in-memory 
representation of a LineString, and its topological, point-set meaning.  
In the textual representation repeated points can occur, but in the 
topological representation repeated points have no meaning.

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022