[geos-devel] Issues with relate not handling GeometryCollections?

chodgson at refractions.net chodgson at refractions.net
Thu Dec 5 12:10:19 EST 2002

I believe there are a couple of PostGIS functions which just take the first 
geometry in the collection, and do the operation on that, in the case that 
handling a collection is either too difficult or meaningless. At the PostGIS 
level, we would want to post a NOTICE or WARNING or something like that.


Quoting Martin Davis <mbdavis at VividSolutions.com>:

> Here's an issue which is really more of a general JTS issue, but it may
> impact the integration with PostGIS, so I thought I'd throw it out here:
> Currently JTS does NOT handle GeometryCollections as input to relate. This is
> for two reasons:
> (i) the SFS did not define the semantics for this
> (ii) the semantics which I suspect are most useful are to treat the GC as the
> union of its components.  Unfortunately, I don't know how to compute this
> robustly (either implicitly or explicitly).  (Well, I do, but it's a LOT of
> work).  This is annoying, since you really want relate to be robust AND
> exact.  Since I couldn't do this, I chose to make GC's an invalid argument to
> relate.
> The question is, is this a big deal for PostGIS?  It *is* a pain not having
> operators complete over the entire space of representable objects.'
> There is a cheesy way of defining relate over GCs, to be the "sum" of relate
> over the individual components.  This works for intersection (an important
> case) but produces counterintuitive results for some of the other predicates.
> Thoughts, anyone?
> Martin Davis, Senior Technical Specialist
> Vivid Solutions Inc.
> Suite #1A-2328 Government Street   Victoria, B.C.   V8T 5G5
> Phone: (250) 385 6040    Fax: (250) 385 6046
> EMail: mbdavis at vividsolutions.com  Web: www.vividsolutions.com
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