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

Norman Vine nhv at cape.com
Wed Dec 4 21:30:50 EST 2002

Martin Davis writes:
> 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?

I was mildly surprised that there is no spatial partitioning in GEOS.

i.e. subdividing the elements into pieces coresponding to their
spatial bucket, performing the 'spatial operations' on the pieces
within each bucket, and then reassembling the resulting objects.

Once the problem space gets big enough something similar
is needed any way or you are going to run out of core.



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