[postgis-users] Re: Buffering a multipolygon gives Empty Geometry.

[rm_postgis at cheapcomplexdevices.com]

Paul Ramsey <pramsey at refractions.net> wroe:
> 
> OK, here is the odd bit about your examples: they are both buffer(0) 
> cases.

The buffer(0) example was the test-case that I could isolate
in something small enough to post to the mailing list.  I can find
where the "geomunion" crashes with positive polygons


> What are you trying to accomplish with this?

A generic user-defined interface using mapserver-CGI that lets the end 
user say "give me all events withing X miles of the Florida A&M campus".
If the end user happens to say '0', it ran the same query as if he said
1 mile that looks something like:

   geomunion(buffer(the_geom,X))

> That said, they should still work, so perhaps our buffer(0) semantics in 
> GEOS far fubar.
	
With various geometries I can have geomunion crash on positive-sized
buffers too; but the first one I saw had a zero, so I was thinking
the empty geometry collection might have caused the geomunion to crash.

> I found that example 1 worked with a positive buffer, but with 0 my test 
> platform gave the same results as yours, an empty geometrycollection.

I can post larger examples (a dozen large polygons) that crash the backend 
but haven't been on the mailinglist long enough to know the etiquite of 
posting large exmaples..  Perhaps I'll put together a SQL script and
post a link to it on the internet that show the problem with non-zero
buffer sizes if that would be of interest here.

   Ron