I have looked at the special case of a raster combining situation 3 and 4 below:
3 + 4: # # # # # # # # The polygoniser handles this fine by return a result with two separate polygons. For easier reference, I have included the WKT of the 4 + 1 situations and their results: 1: Input: POLYGON ((1 1,3 1,3 2,2 2,2 3,3 3,3 2,4 2,4 4,1 4,1 1)) Result: POLYGON ((1 4,1 1,3 1,3 2,2 2,2 3,3 3,3 2,4 2,4 4,1 4)) 2: Input: POLYGON ((1 1,4 1,4 3,3 3,3 2,2 2,2 3,3 3,3 4,1 4,1 1)) Result: POLYGON ((1 4,1 1,4 1,4 3,3 3,3 2,2 2,2 3,3 3,3 4,1 4)) 3: Input: POLYGON ((1 1,4 1,4 4,2 4,2 3,3 3,3 2,2 2,2 3,1 3,1 1)) Result: POLYGON ((2 4,2 3,1 3,1 1,3 1,4 1,4 4,2 4),(2 3,2 2,3 2,3 3,2 3)) 4: Input: POLYGON ((1 4,1 2,2 2,2 3,3 3,3 2,2 2,2 1,4 1,4 4,1 4)) Result: POLYGON ((1 4,1 2,2 2,2 1,4 1,4 4,1 4),(2 3,3 3,3 2,2 2,2 3)) 3 + 4: Input: MULTIPOLYGON (((1 2,2 2,2 4,1 4,1 2)),((2 1,4 1,4 5,2 5,2 4,3 4,3 2,2 2,2 1))) Result: POLYGON ((1 4,1 2,2 2,2 4,1 4)) + POLYGON ((2 5,2 4,3 4,3 2,2 2,2 1,3 1,4 1,4 5,2 5)) I have performed the tests on both GDAL 1.11 and GDAL 2.1 and both return the same results. Can anyone confirm my observations? Background: The 4 situations are very simplified versions of much more complicated polygons, where the problem occurs. If I save the results from situation 3 or 4 to a shape file with ogr, the hole in the results are converted to an exterior ring in a separate polygon thus creating overlapping results. Regards, Casper From: gdal-dev [mailto:gdal-dev-boun...@lists.osgeo.org] On Behalf Of Casper Børgesen (CABO) Sent: 19. januar 2017 13:26 To: gdal-dev@lists.osgeo.org Subject: [gdal-dev] Strange results of simple polygonising Hi, I have four simple rasters that I would like to polygonise: 1: # # # # # # # 2: # # # # # # # 3: # # # # # # # 4: # # # # # # # It's the same shape, just rotated 90, 180, 270 degrees. The resulting polygons for 1 and 2 are simple polygons without holes, where the results for 3 and 4 are polygons with a hole intersecting the exterior ring. Thus 3 and 4 results in invalid geometries with self-intersection. Is this the intended behavior of the polygoniser? Regards, Casper
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