[geos-devel] MULTI* and Invalid

[Martin Davis]

The relevant section of the OGC SFS spec (99-049) is 2.1.11 MultiSurface:

"The boundaries of any two elements in a MultiSurface may intersect at 
most at a finite number of points"

An intersection which is a line segment contains an infinite number of 
points, so this is invalid.

On 6/14/2013 6:41 AM, Sandro Santilli wrote:
> On Fri, Jun 14, 2013 at 01:16:02PM +0200, Oliver Courtin wrote:
>> Hi,
>>
>>
>> Below an example, throught PostGIS
>> with GEOS 3.3.5-CAPI-1.7.5:
>>
>>
>> SELECT ST_IsValidReason(
>>    'MULTIPOLYGON(((0 0,1 0,1 1,0 1,0 0)),((1 0,2 0,2 1,1 1,1 0)))'::geometry
>>    );
>>
>>
>> Output:   Self-intersection[1 0]
>>
>>
>> After looking in SFS and ISO 19107,
>> we didn't find at first sight
>> why such a geometry should be invalid.
>>
>>
>> Could you give us some light on it ?
> The two components have a lineal intersection.
> Puntal intersection would be considered valid
> (ie: moving vertex  1,1  to 1,1.5).
>
> IIRC there was a "single point" wording in the spec, but
> Martin will know more about this.
>
> --strk;
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