plugins_geometry_checker.rst

.. index:: Digitizing, Topology, Geometry validity, Errors
   single: Plugins; Geometry checker

Geometry Checker Plugin

Geometry Checker is a powerful core plugin to check and fix the geometry validity of a layer. It is available from the :menuselection:`Vector` menu ( :menuselection:`Check Geometries...`).

Configuring the checks

The :guilabel:`Check Geometries` dialog shows different grouped settings in the first tab (:guilabel:`Setup`):

• :guilabel:`Input vector layers`: to select the layers to check. A :guilabel:`Only selected features` checkbox can be used to restrict the checking to the geometries of the selected features.
• :guilabel:`Allowed geometry types` gives the chance to restrict the geometry type of the input layer(s) to:
  • Point
  • Multipoint
  • Line
  • Multiline
  • Polygon
  • Multipolygon
• :guilabel:`Geometry validity`. Depending on geometry types you can choose between:
  • :guilabel:`Self intersections`
  • :guilabel:`Duplicate nodes`
  • :guilabel:`Self contacts`
  • :guilabel:`Polygon with less than 3 nodes`.
• :guilabel:`Geometry properties`. Depending on geometry types, different options are available:
  • :guilabel:`Polygons and multipolygons may not contain any holes`
  • :guilabel:`Multipart objects must consist of more than one part`
  • :guilabel:`Lines must not have dangles`
• :guilabel:`Geometry conditions`. Allows you to add some condition to validate the geometries with:
  • :guilabel:`Minimal segment length (map units)`
  • :guilabel:`Minimum angle between segment (deg)`
  • :guilabel:`Minimal polygon area (map units sqr.)`
  • :guilabel:`No sliver polygons` with a :guilabel:`Maximum thinness` and a :guilabel:`Max. area (map units sqr.)`
• :guilabel:`Topology checks`. Depending on geometry types, many different options are available:
  • :guilabel:`Checks for duplicates`
  • :guilabel:`Checks for features within other features`
  • :guilabel:`Checks for overlaps smaller than`
  • :guilabel:`Checks for gaps smaller than`
  • :guilabel:`Points must be covered by lines`
  • :guilabel:`Points must properly lie inside a polygon`
  • :guilabel:`Lines must not intersect any other lines`
  • :guilabel:`Lines must not intersect with features of layer`
  • :guilabel:`Polygons must follow boundaries of layer`
• :guilabel:`Tolerance`. You can define the tolerance of the check in map layer units.
• :guilabel:`Output vector layer` gives the choice to:
  • :guilabel:`Modify input layer`
  • :guilabel:`Create new layers`

When you are happy with the configuration, you can click on the :guilabel:`Run` button.

The Geometry Checker Plugin

The Geometry Checker Plugin can find the following errors:

• Self intersections: a polygon with a self intersection
• Duplicate nodes: two duplicates nodes in a segment
• Holes: hole in a polygon
• Segment length: a segment length lower than a threshold
• Minimum angle: two segments with an angle lower than a threshold
• Minimum area: polygon area lower than a threshold
• Silver polygon: this error come from very small polygon (with small area) with a large perimeter
• Duplicates features
• Feature within feature
• Overlaps: polygon overlapping
• Gaps: gaps between polygons

The following figure shows the different checks made by the plugin.

Some checks supported by the plugin

Analysing the results

The results appear in the second tab (:guilabel:`Result`) and as an overview layer of the errors in the canvas (its name has the default prefix :file:`checked_`). A table lists the :guilabel:`Geometry check result` with one error per row and columns containing: the layer name, an ID, the error type, then the coordinates of the error, a value (depending on the type of the error) and finally the resolution column which indicates the resolution of the error. At the bottom of this table, you can :guilabel:`Export` the error into different file formats. You also have a counter with the number of total errors and fixed ones.

You can select a row to see the location of the error. You can change this behavior by selecting another action between :guilabel:`Error` (default), :guilabel:`Feature`, :guilabel:`Don't move`, and :guilabel:`Highlight selected features`.

Below the zoom action when clicking on the table row, you can:

• :guilabel:`Show selected features in attribute table`
• :guilabel:`Fix selected errors using default resolution`
• :guilabel:`Fix selected errors, prompt for resolution method` You will see a window to choose the resolution's method among which:
  • Merge with neighboring polygon with longest shared edge
  • Merge with neighboring polygon with largest area
  • Merge with neighboring polygon with identical attribute value, if any, or leave as is
  • Delete feature
  • No action
• :guilabel:`Error resolution settings` allows you to change the default resolution method depending on the error type

Tip

Fix multiple errors

You can fix multiple errors by selecting more than one row in the table with the CTRL + click action.

Finally, you can choose which :guilabel:`Attribute to use when merging features by attribute value`.