[geos-devel] bad input data or robustness issue?

Russell Strong russell at strong.id.au
Wed Mar 5 04:08:02 EST 2008

I've had a look through the c api and it would appear that access to 
precision models is not exposed, unless it's disguised in a way I have 
not recognized.  Am I correct?  Is there some code that someone knows 
about which could give me some *pointers.

Martin Davis wrote:
> As you've noticed, the predicates are not necessarily consistent with 
> the overlay operations.  This is because the predicates are exact, 
> whereas the overlay operations are approximate (which is unavoidable, 
> since they operated in a finite-precision model).
> So, don't rely on this in your code. If you determine that two 
> polygons overlap, you still have to check for an empty intersection 
> and handle it appropriately.  This should only happen when the area of 
> overlap is so small as to be negligible, in any case.
> Also, try using a limited-precision model.  Round of the numbers in 
> your input, and use an explicit PrecisionModel to control the 
> precision of the computed output.  This should increase stability.
> It might also help if you checked the area of intersections and 
> differences and eliminated ones with very small areas (since these 
> should be below the accuracy of your input data in any case).
> HTH - Martin
> Russell Strong wrote:
>> Hi all,
>> I'm trying to dissect up a bunch of nearly round overlapping 
>> polygons, ie.. trying to find areas of overlapping radar coverage 
>> given a set of radar locations and ranges.
>> However, trying this with a few polygons, testing each case I can 
>> think of works very well.  Add a few more and I get all sorts of 
>> errors including:
>> * side location conflict
>> * stuck in endless loop ( due to intersect and difference operations 
>> that produce the same  polygons as we started with )
>> * non-noded intersection
>> * no outgoing dirEdge found
>> I've included some test code that shows all of these,  ( uncomment 
>> various tests in main ).  If anyone has some time to run these and 
>> comment I'd appreciate it.  I've spent 5 solid days on it and I'm out 
>> of ideas.
>> it basically works like this:
>> I keep a link list of "areas" which contain a geometry.  I then 
>> compare each geometry against each other ( except for self ).  If it 
>> intersects, I add the intersection and the 2 differences to the list 
>> (if they exist) and remove the source areas.  I keep going until I've 
>> compared every area against every other area and found no non-empty 
>> intersections.
>> One other thing that I found was the Overlaps can return true but the 
>> intersection of the two polygons returns true for isEmpty?!?!?  (I 
>> was using overlaps instead of intersects in the split_areas comparison)
>> Thanks,
>> Russell
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