24.2.3. 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 Vector menu (geometryChecker Check Geometries…).

24.2.3.1. Configuring the checks

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

  • Input vector layers: to select the layers to check. A checkbox Only selected features checkbox can be used to restrict the checking to the geometries of the selected features.

  • Allowed geometry types gives the chance to restrict the geometry type of the input layer(s) to:

    • checkbox Point

    • checkbox Multipoint

    • checkbox Line

    • checkbox Multiline

    • checkbox Polygon

    • checkbox Multipolygon

  • Geometry validity. Depending on geometry types you can choose between:

    • checkbox Self intersections

    • checkbox Duplicate nodes

    • checkbox Self contacts

    • checkbox Polygon with less than 3 nodes.

  • Geometry properties. Depending on geometry types, different options are available:

    • checkbox Polygons and multipolygons may not contain any holes

    • checkbox Multipart objects must consist of more than one part

    • checkbox Lines must not have dangles

  • Geometry conditions. Allows you to add some condition to validate the geometries with:

    • checkbox Minimal segment length (map units) selectNumber

    • checkbox Minimum angle between segment (deg) selectNumber

    • checkbox Minimal polygon area (map units sqr.) selectNumber

    • checkbox No sliver polygons with a Maximum thinness selectNumber and a checkbox Max. area (map units sqr.) selectNumber

  • Topology checks. Depending on geometry types, many different options are available:

    • checkbox Checks for duplicates

    • checkbox Checks for features within other features

    • checkbox Checks for overlaps smaller than selectNumber

    • checkbox Checks for gaps smaller than selectNumber

    • checkbox Points must be covered by lines

    • checkbox Points must properly lie inside a polygon

    • checkbox Lines must not intersect any other lines

    • checkbox Lines must not intersect with features of layer selectString

    • checkbox Polygons must follow boundaries of layer selectString

  • Tolerance. You can define the tolerance of the check in map layer units.

  • Output vector layer gives the choice to:

    • radioButtonOn Modify input layer

    • radioButtonOn Create new layers

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

../../../../_images/check_geometries.png

Fig. 24.11 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.

../../../../_images/geometry_checker_scheme.png

Fig. 24.12 Some checks supported by the plugin

24.2.3.2. Analysing the results

The results appear in the second tab (Result) and as an overview layer of the errors in the canvas (its name has the default prefix checked_). A table lists the 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 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 radioButtonOn Error (default), radioButtonOff Feature, radioButtonOff Don’t move, and checkbox Highlight contour of selected features.

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

  • fromSelectedFeature Show selected features in attribute table

  • success Fix selected errors using default resolution

  • success 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

  • options 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 Attribute to use when merging features by attribute value.