# [geos-devel] Binary Predicate Bug - Even Worse!

Todd Jellett todd.jellett at caris.com
Tue Jun 12 13:01:28 EDT 2007

```Read the post Paul. I very aware of what the spec is and says.

the inverse of disjoint(), intersects(), the Binary Predicate.

A.equals(B) = TRUE implies that A.intersects(B) = TRUE
A.touches(B) = TRUE implies that A.intersects(B) = TRUE
A.contains(B) = TRUE implies that A.intersects(B) = TRUE
A.within(B) = TRUE implies that A.intersects(B) = TRUE
A.overlaps(B) = TRUE implies that A.intersects(B) = TRUE

on the other hand this *does not* work the same way for contains/within

A.equals(B) = TRUE *does not imply* that A.within(B) = TRUE
A.touches(B) = TRUE *does not imply* that A.within(B) = TRUE
A.contains(B) = TRUE *does not imply* that A.within(B) = TRUE
A.disjoint(B) = TRUE *does not imply* that A.within(B) = TRUE

So I excluded the binary predicate intersects() to simplify my example
(which you seem to have missed altogether).

My example has a simple 1-ring 5-point polygon that is a square. When
this geometry is tested against itself by calling each of the binary
predicates in turn, I observe that A.equals(B) = TRUE, A.contains(B) =
TRUE *and* A.within(B) = TRUE. This is what I am questioning the
validity of.

Nowhere, absolutely nowhere in the OGC SFSQL does it say that a single
geometry (any two geometries for that matter) can be equal to each
other, and at the same time have A be contained in itself (or another
geometry B) *and* have A within itself (or another geometry B).

If this is how it is supposed to be then the equals() predicate is
redundant and could be eliminated. (equal = contains && within).

Todd

Paul Ramsey wrote:
> The OGC SFSQL document says that
>
> A.Within(B) implies Insersection(A,B) == A
>
> And Contains is just defined for commutative purposes against Within():
>
> A.Within(B) implies B.Contains(A).
>
> So, you might not like the semantics, but they are implemented as
> defined by the standards body.
>
> Paul
>
> Todd Jellett wrote:
>> It turns out that this is also the case for identical geometries!
>>
>> If you take just GeomA and run all the listed binary predicates
>> (below) against itself, you get exactly the same as below.
>>
>> Running GeomA->GeomA I get:
>> Disjoint    False
>> Equal       True
>> Touch      False
>> Contain    True
>> Within      True
>> Overlap    False
>>
>> Running a simple geometry against itself should return True for
>> Equals *only*. It is ambiguous to be also contained and within.
>>
>> Todd
>

```