[postgis-users] Nitpicking on empty geometries

Martin Davis mbdavis at refractions.net
Thu Jun 12 09:41:42 PDT 2008

I don't think that the SFS requires GeometryCollections to be 
non-empty.  If you think it does, can you provide a page & section number?

The reason that intersection() returns GC EMPTY as its null return value 
is that this is the standard value that JTS/GEOS uses to indicate null 
returns.  It was thought that it was better to have a single "null" 
value, rather than trying to return an empty geometry of a different 
type.  (For instance, what would be the null return value for 
intersection(POLYGON, LINESTRING)?)

However, this is a somewhat arbitrary design decision, and it may be 
that it would better to always have intersection etc. return "the most 
correct type".  Or perhaps it should return the type of smallest dimension?

This would have the advantage that overlay ops would then always return 
a value which could be used in further overlay ops.

Any comments?

Rolf A. de By wrote:
> Greetings all,
> My st_intersection() of two multipolygons returns an empty geometry: 
> no problems, as it should.  When I ask for its st_geometrytype() I am 
> told it is "ST_Geometry", and I can live with that.  When I ask for 
> its st_astext() I get "GEOMETRYCOLLECTION EMPTY," and that by itself I 
> can also live with.  However, in combination I find those answers 
> inconsistent and potentially confusing for writing geometry 
> manipulation code.  And also, isn't the OGC standard SF99 demanding 
> that geometry collections always have at least one member geometry?  
> Or am I missing something here?
> Rolf
> International Institute for Geo-Information Science and Earth 
> Observation (ITC)
> Chamber of Commerce: 410 27 560
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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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