ftools.dissolve and geoprocessing.dissolve "loose" polygons #12866
Comments
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Author Name: Giovanni Manghi (@gioman)
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Author Name: Paolo Cavallini (@pcav)
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Author Name: Giovanni Manghi (@gioman) The vector is in much bad conditions than it seem just looking at it (and the QGIS "check geometry" tool basically looks only for self-intersections as in the pseudo-topological model are both allowed holes and overlapping areas). Importing it into GRASS it shows that there are 610 holes, 310 overlapping areas and a number of 0 size areas. If you dissolve the vector with GRASS, or with QGIS after exporting 1_polygon from GRASS (as shapefile, for example), you'll get the "expected" result, but the 0 size areas will show very clearly in the middle of the polygons (see attached image). The solution is importing into GRASS and during the operation setting the parameters "snapping threshold for boundaries" (to remove the slivers/holes) and "minimum size area to be imported" (to remove 0 size areas). This way the resulting vector will be perfectly clean (attached) and it will dissolve with no problems. You may want also to read this ticket, especially the last two comments in #13186 (comment) Resuming: Arc* does some kind tolerance/snapping when dissolving, GEOS (the engine QGIS uses) doesn't (to keep as much precision as possible during union operations), but the option exist and may make sense to expose in the QGIS gui. You may want to open a feature ticket about this. This is the result of the import operation of the original vector into GRASS is the following @v.in.ogr -o dsn=/home/gio/Desktop/suelosenah.shp output=suelo snap=-1 min_area=0.0001 Over-riding projection check Layer: suelosenah Counting polygons for 63 features... Importing map 63 features... Building topology for vector map ... Registering primitives... 761 primitives registered 13901 vertices registered Number of nodes: 741 Number of primitives: 761 Number of points: 0 Number of lines: 0 Number of boundaries: 761 Number of centroids: 0 Number of areas: - Number of isles: - Cleaning polygons, result is not guaranteed! Break polygons: Remove duplicates: Break boundaries: Remove duplicates: Clean boundaries at nodes: Break boundaries: Remove duplicates: Clean boundaries at nodes: Change dangles to lines: Remove bridges: Merge boundaries: Building topology for vector map ... Building areas... Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored 1038 areas built 59 isles built Attaching islands... Number of nodes: 9238 Number of primitives: 29041 Number of points: 0 Number of lines: 0 Number of boundaries: 29041 Number of centroids: 0 Number of areas: 1038 Number of isles: 59 Number of incorrect boundaries: 78 Number of areas without centroid: 1038 Cannot calculate area centroid Cannot calculate area centroid Cannot calculate area centroid Cannot calculate area centroid Cannot calculate area centroid Cannot calculate area centroid Cannot calculate area centroid Cannot calculate area centroid Cannot calculate area centroid Find centroids for layer: suelosenah Write centroids: 407 areas represent more (overlapping) features, because polygons overlap in input layer(s). Such areas are linked to more than 1 row in attribute table. The number of features for those areas is stored as category in layer 2 63 input polygons Total area: 1.49304E+08 (1038 areas) Overlapping area: 32.6961 (407 areas) Area without category: 1262.18 (400 areas) Building topology for vector map ... Registering primitives... 17787 vertices registered Building areas... Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored Area of size = 0.0 ignored 1038 areas built 59 isles built Attaching islands... Attaching centroids... Number of nodes: 2839 Number of primitives: 3951 Number of points: 0 Number of lines: 0 Number of boundaries: 3322 Number of centroids: 629 Number of areas: 1038 Number of isles: 59 Number of incorrect boundaries: 78 Number of duplicate centroids: 201 Number of areas without centroid: 610
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Author Name: Giovanni Manghi (@gioman)
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Author Name: Giovanni Manghi (@gioman) The "ftools" category is being removed from the tracker, changing the category of this ticket to "Processing/QGIS" to not leave the category orphaned. |
qgib
added
Bug
Author Name: Anita Graser (@anitagraser)
Original Redmine Issue: 2806
Redmine category:processing/qgis
Both dissolve tools fail on the attached shapefile. Polygons are lost in the process. Compare http://forum.qgis.org/viewtopic.php?f=2&t=6724
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