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I am a new QGIS 2.0 user. I was so impressed the new default tool, Topology Checker, in this new version.

But, hundreds errors are bothering me for cleaning up features. Is it possible to clean errors automatically, not manually?


I have an additional 2 questions.

  1. Is it possible to clean topological error in QGIS not GRASS?
  2. According to "v.clean tool", is it possible to clean overlapped areas? In my understanding, there is no specific tool for overlap error.

3 Answers 3

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You can use the v.in.ogr GRASS module. See Fixing Invalid Geometries With Quantum GIS


Regarding to your new questions, there is a phrase in the above link: Fire up the QGIS/GRASS plugin (or the GRASS native GUI if you prefer) and open/create a mapset and open the GRASS toolbox.

As you can see, it possible to clean topological error directly in QGIS (but using the GRASS Tools plugin). The most interesting is the fact you can use both v.in.ogr and v.clean modules, and a lot of useful tools, also.

These are the steps, for you to be able to access these modules:

  • Install the GRASS plugin in QGIS by going to Plugins > Manage and Install Plugins.... Select GRASS, install it and activate it.

  • Go to Plugin > GRASS > New mapset and follow all those steps to create and save your mapset.

  • Finally, access Plugins > GRASS > Open GRASS Tools, and in the Modules tab, search for the desired modules:

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  • @Yousuke I've updated my answer. Feb 3, 2014 at 14:17
  • I imported 187 multiline features using the v.in.org.gis module and grass outputted 328 features. It appears to have broke lines that were merged into multi lines. Any idea why it does this? I then used v.clean.snap with a threshold of 5 and it worked great and brought my features down to 305. I think the topology is solid but worried about data integrity with the duplicates?
    – pdog
    Nov 19, 2016 at 1:27
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There is grass tools, v.clean, that include tools for fixing the majority of topological errors automatically. Below you find the list of what it does :

break: break lines at each intersection
snap: snap lines to vertex in threshold
rmdangle: remove dangles, threshold ignored if < 0
chdangle: change the type of boundary dangle to line, threshold ignored if < 0, input line type is ignored
rmbridge: remove bridges connecting area and island or 2 islands
chbridge: change the type of bridges connecting area and island or 2 islands from boundary to line
rmdupl: remove duplicate geometry features (pay attention to categories!)
rmdac: remove duplicate area centroids ('type' option ignored)
bpol: break (topologically clean) polygons (imported from non topological format, like ShapeFile). Boundaries are broken on each point shared between 2 and more polygons where angles of segments are different
prune: remove vertices in threshold from lines and boundaries, boundary is pruned only if topology is not damaged (new intersection, changed attachment of centroid), first and last segment of the boundary is never changed
rmarea: remove small areas, the longest boundary with adjacent area is removed
rmline: remove all lines or boundaries of zero length, threshold is ignored
rmsa: remove small angles between lines at nodes
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You can use v.clean or Check Geometry plugin to clean topology error in QGIS. But must be kept in mind, using the tools not always working perfectly for any cases, but at least it could save time and speed up the process in cleaning topology error. This tutorial explains how to use v.clean and check geometry to clean topology error

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