[postgis-devel] Spatial relationships not defined should return null

[Sandro Santilli]

On Fri, Feb 10, 2012 at 12:59:54PM +0100, Jose Carlos Martinez wrote:
> On 10/02/2012 12:05, Mark Cave-Ayland wrote:
> >On 10/02/12 04:53, Paul Ramsey wrote:
> >
> >>I'm not enthusiastic to change this behavior.
> >>P.
> >>
> >>On Thu, Feb 9, 2012 at 2:39 PM, Jose Carlos Martinez Llario
> >><jomarlla at cgf.upv.es> wrote:
> >>>Hi list,
> >>>According to the standards the spatial predicates which are
> >>>not defined as
> >>>st_touches (point , point) or st_overlaps (line, polygon) must
> >>>return null.
> >>>PostGIS returns false in those cases (trunk and previous
> >>>versions too), so
> >>>it is against SQL/MM.
> >>>
> >>>Should I open a ticket or this is the wanted behavior?
> >>>
> >>>Regards,
> >>>Jose
> >
> >I agree with Paul, it does sound a little strange. Can you point
> >us towards the relevant page in SQL/MM? Also what do SQL
> >Server/Oracle Spatial do?
> >
> >
> >ATB,
> >
> >Mark.
> >
> Hi, for me is not strange, in fact returning false is so confusing
> because the user can think the spatial relationship is defined. Im
> sure many users already have made this kind of mistake.
> The SQL/MM is clear about that:
> 
> ISO 13249-3:2011 -> 4.2.2.3 Spatial Methods using ST_Geometry
> 
> one example (I copied the test from the standard maybe this is not
> ok because of the copyright, sorry):
> 
> "ST_Touches:
> 
> ST_Touches: tests if an ST_Geometry value spatially touches another
> ST_Geometry value.
> ISO/IEC 13249-3:2011(E)
> 20 © ISO/IEC 2011 - All rights reserved
> Given two geometries a and b, ST_Touches is defined as:
> Case:
> a) If a.ST_Dimension() = 0 (zero) and b.ST_Dimension() = 0 (zero),
> then the null value
> b) Otherwise, a.ST_Touches(b) ⇔ (Interior(a) ∩ Interior(b) = ∅) ∧ (a
> ∩ b) ≠ ∅"
> 
> 
> I think PostGIS is making an effort in the 2.0 release to follow
> standards in a more strict way, but Lately I dont understand the
> PostGIS policy about standards, sometimes PostGIS follows standards
> sometimes doesnt as I already discussed with the ST_StartPoint,
> ST_GeometryN, etc. in a previous email. and what is worse from my
> point o view sometimes without a logical reason.

I guess the same reasoning should take us returning NULL
for ST_Equals(empty,empty) ? or is that unspecified ?

I personally don't value unapplied standards that much.

--strk; 

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