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I'm working on bug #1314 (Exception on buffering polygon shapefile) and I think I found an anomaly in the NTS Library. Here's the note I've added to the bug: Quote
[NTS Library] Help with debugging
Posted by: pmeems ()
Date: July 23, 2009 05:02AM

I'm working on bug #1314 (Exception on buffering polygon shapefile) and I think I found an anomaly in the NTS Library.

Here's the note I've added to the bug:
Quote

I've been working on this bug and I found some polygon are converted by the NTS library to a Geometry collection instead of a polygon. Converting this back results in a NULL_SHAPE and the exception posted.

In MapWinGeoProc.NTS.Topology.Operation.Overlay.OverlayOp.ComputeOverlay
the count for resultLineList > 0 so resultGeom = ComputeGeometry(resultPointList, resultLineList, resultPolyList) has a line and a polygon and that is why a Geometry collection is created.
I don't understand why a line has been found. MapWindow thinks it is a polygon as it should.

Next I started buffering each polygon separately by selecting just one each time.
This sometimes resulted in a 'side location conflict' in MapWinGeoProc.NTS.Topology.GeometriesGraph.EdgeEndStar.PropagateSideLabels
If I move a point of the bad polygon using the shapefile editor and try again the shape is correctly buffered. So something is wrong with the shape and accidentally the shapefile editor is fixing it.
The problem seems to be with the NTS library and not with MapWindow. The shapefile is used fine with ArcGIS.

So if anyone has some more suggestions, please let me know.

Thanks,
Paul

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Paul Meems
The Netherlands
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Release manager, configuration manager and
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