#754: Deep Vector Bug: Overlay functionality flawed in v.select and v.in.ogr -----------------------+---------------------------------------------------- Reporter: ploewe | Owner: [email protected] Type: defect | Status: new Priority: critical | Milestone: 6.5.0 Component: Vector | Version: 6.4.0 RCs Resolution: | Keywords: v.overlay Platform: Linux | Cpu: Unspecified -----------------------+---------------------------------------------------- Comment (by mmetz):
Replying to [comment:3 ploewe]: > Replying to [comment:2 mmetz]: > > Replying to [comment:1 martinl]: > > > > Sample data / Screenshots can be produced on request. > > > > > > Yes, please attach sample data e.g., in GRASS ASCII vector format. > > > > > +1, if the sample data are too large for attachment, please send them off-list as attachment. > > > > Could you also provide information about what were the exact commands you used, what do you want to happen with the overlapping areas i.e. which cleaning operations should be performed, and what should the desired result look like. > > > > Done. > Two sample input data sets were attached as ESR! shapes to the ticket plus the result from v.patch as generated by grass6.4R5. > Here's the challenge: The two tile grids provided in the two input shapefiles partially overlap. The task is NOT to clean the topology (and to create new sub-polygons in the overlap area), BUT to maintain the current polygon pattern, which implies that a number of poylgons overlap. AFAICT, this is not supported by the GRASS vector topology model by design. In order to attach a centroid to an area, at least some part of that area must not be overlapped by any other area, otherwise there will be areas without centroids and/or areas with duplicate centroids. In your patched example, there are a number of areas that are completely overlapped by other areas. There, centroids can neither be calculated nor properly attached. Patching these two vectors together and maintaining areas exactly as they are, with their category and attribute link, works only in a non-topological vector environment. Partially overlapping polygons are supported by GRASS, e.g. Landsat tiles can be imported and maintained as they are if they are not cleaned, because tiles overlap only at the edges and centroids can be properly attached. As long as there is a small bit of the area not overlapped by another area, you can manually move the centroid to there to remove duplicate centroids. This is however impossible for a number of areas in the overlap zone. Markus M -- Ticket URL: <https://trac.osgeo.org/grass/ticket/754#comment:4> GRASS GIS <http://grass.osgeo.org>
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