[postgis-devel] Should EMPTY be spatially equal to self ?

[Sandro Santilli]

On Tue, Jan 17, 2012 at 03:31:10PM +0100, Havard Tveite wrote:

> I agree with strk regarding the use of the Simple Feature for
> SQL Specification as the "bible".  Being what PostGIS is, we
> should do our best to do the "right thing", and then try to
> convince those responsible for the specifications and standards
> to do the same.  Then the other players will come to their
> senses. ;-)
> 
> Spatial joins / spatial predicates in SQL/MM Spatial are based
> on the DE9IM.  How sound is this foundation?

Pretty much. Internally, the predicates do use DE9IM, except
for some optimizations implemented _before_ computing the whole
intersection matrix. Optimizations currently include bounding
box comparosons.

> FFF.FFF.FF2 is a sensible DE9IM matrix for empty - empty in
> set theoretical terms, but does it make sense to apply it for
> spatial predicates?  Many say yes, some say no.

I don't think anyone is suggesting NOT to use the matrix in
spatial predicates. I think what I'm suggesting to change
the definition of ST_Equals in matrix terms, or to introduce
an explicit corner case for the EMPTY case.

--strk;

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