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

Stephen Woodbridge woodbri at swoodbridge.com
Wed Mar 5 11:51:30 EST 2008

Would this make a good Google SoC project? I seems like it would be a 
good thing to have in the c-api.


Martin Davis wrote:
> Er, well, perhaps not.  Can you use the C++ API instead?  Or else this 
> is a project for some GEOS developer...
> Russell Strong wrote:
>> 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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