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

[Sandro Santilli]

[postgis-devel back in the loop]

On Mon, Jan 16, 2012 at 02:19:58PM -0800, Martin Davis wrote:

> - According to the SFS, the DE9IM entry for the empty set is F.  So I agree
> with Sandro that the DE9IM for two empty geometries is:
> 
> FFF.FFF.FF2
> 
> (since the intersections I/I, B/B, and E/E are all empty)
> 
> According to the SF definition of Equals (TFF.FTF.FFT) this means that
> Equals(EMPTY, EMPTY) = false.  Maybe not the most desirable result in some
> situations, but it's good to be consistent.  After all, that's the whole
> reason for defining the predicates in terms of the DE9IM in the first place.

Speaking of consistency, this is the DE9IM of a point to itself:

       0FF.FFF.FF2

It doesn't match the SF definition of Equals (B/B is false).
Should a point not be equal to itself for the sake of being consistent ?

The SFS standard is short on this topic, as well as on the topic of
empties in general. I wouldn't hung on following it literally.

--strk; 

  ()   Free GIS & Flash consultant/developer
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