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

Martin Davis mbdavis at refractions.net
Wed Mar 5 11:46:21 EST 2008

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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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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