[geos-devel] Any sneaky tricks for speeding up operations with complex geometries?

[Martin Davis]

>
>
>
> > About the union question, probably no good news there, unless your data
> has some very unlikely characteristics.  The GEOS Cascaded Union approach
> is remarkably efficient at unioning sets of overlapping polygons - which it
> sounds like you have.  The other alternative hack is to run buffer(0), but
> it is unlikely to be faster.
>
> Ok, thanks for the confirmation. Is there any benefit in "batching"
> sets of geometries to cascaded union (e.g. unioning 100 geometries in
> sets of 10, and then doing a final union of the result)? Or best to
> throw EVERYTHING at GEOS and let it sort it out?
>

Best to let GEOS handle it.  It uses a spatial index to choose sets of
polygons to union. It's unlikely you could do this better (unless there is
some characteristic of the data you can take advantage of)

>
> > IF the data was a true polygonal coverage (i.e. strictly
> non-overlapping) then there is a faster approach in GEOSCoverageUniion.
> [1].  But if you are trying to find gaps then it's likely your data is not
> a true coverage.
>
> Interesting. I have one application which would definitely benefit
> from this. GEOS 3.8 looks to be a very exciting release..! One thing I
> couldn't determine from the coverage PR and the docs of the new
> operation is what happens when the data **isn't** a non-overlapping
> coverage. Is the result total garbage in this case or is an exception
> raised? Are holes/non-contiguous parts allowed?
>
> It looks like GEOSCoverageUnion is able to detect non-fully-noded inputs
[1] and overlaps [2], and throws an exception in this case.

I would think holes and non-contiguous parts are fine as input.

[1]
https://github.com/libgeos/geos/pull/158/files#diff-7cd9f5f9244e77677b80591da3d99207R94
[2]
https://github.com/libgeos/geos/pull/158/files#diff-7cd9f5f9244e77677b80591da3d99207R123
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