Re: [jts-dev] Struggling with polygon topology again. Overlapping inner

[Phil Scadden <p.scadden@xxxxxxxxxx>]

> In TestBuilder are you working with LinearRings or Polygons? If you 
> are using rings then yes, subtracting one ring from another outside it 
> is just going to return the outer ring.  (Rings don't have area).
Outer was definitely polygon, but inner may have been rings.

> When I subtract the first hole polygon from the shell polygon I get a 
> polygon with one hole, as expected.  I'm not sure why the original 
> code doesn't work, however.  Probably something obvious that neither 
> of us is seeing.  Are the intermediate results at all correct?
No. The difference was having no effect. I will have another look at it. 
Should probably fetch the jts source and have a closer look.
> Another way to do this is to form the polygon with the holes (which 
> will be invalid), and then run buffer(0) on it. That has the effect of 
> fixing the topology.  It's quite fast. Not 100% robust, but should be 
> fine if the polygon geometry is "relatively clean" (which is hard to 
> define more precisely!)
I have avoided this because I found I could have a small self-crossing 
with it keeping the small polygon and destroying the main 
geometry.Because I am bulk processing, it can be hard to detect this 
happening. Hadnt tried it for issue of crossing inner polygon though. 
Speed doesnt worry me so much ( I dont care if runs all night), but 
maximizing the accuracy of the reconstruction does.

--
Phil Scadden, Senior Scientist GNS Science Ltd 764 Cumberland St, 
Private Bag 1930, Dunedin, New Zealand Ph +64 3 4799663, fax +64 3 477 5232

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