[GRASS-dev] Re: [GRASS GIS] #754: Deep Vector Bug: Overlay functionality flawed in v.select and v.in.ogr

GRASS GIS trac at osgeo.org
Tue Sep 22 03:27:59 EDT 2009

#754: Deep Vector Bug: Overlay functionality flawed in v.select and v.in.ogr
  Reporter:  ploewe    |       Owner:  grass-dev at lists.osgeo.org
      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>

More information about the grass-dev mailing list