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

[Martin Davis]

Ok, great, if there's a precedent for hacking around the issue in PostGIS that makes it easier!

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


> -----Original Message-----
> From: chodgson at refractions.net [mailto:chodgson at refractions.net]
> Sent: Thursday, December 05, 2002 9:10 AM
> To: GEOS Development List
> Subject: Re: [geos-devel] Issues with relate not handling
> GeometryCollections?
> 
> 
> 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.
> 
> Chris
> 
> 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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> > geos-devel at geos.refractions.net
> > http://geos.refractions.net/mailman/listinfo/geos-devel
> > 
> 
> 
> 
> 
> 
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