[postgis-users] Polygon validity

[Andrea Peri 2007]

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 ?

In a topological world 2 polygon in touch between each other can have a line in common on their boundaries of-course.
Identically two lines can have a point in common beetween they (on their boundary of course), 
phisically that point can be is the start point or the end point of the line,
but if the start point or the end point are 2 or more point because there are repeated points I think 
is a problem even topologically.

Andrea.