[geos-devel] bad input data or robustness issue?
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
>>> geos-devel mailing list
>>> geos-devel at lists.osgeo.org
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Senior Technical Architect
Refractions Research, Inc.
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