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…).

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: to allow only some geometry types like point, multipoint, line, multiline, polygon and multipolygon.

  • Geometry validity: depending on geometry types, the user can choose checkbox Self intersections, checkbox Duplicate nodes, checkbox Self contacts and checkbox Polygon with less than 3 nodes.

  • Geometry properties: depending on geometry types, the user can choose checkbox Polygons and multipolygons may not contain any holes, checkbox Multipart objects must consist of more than one part and checkbox Lines must not have dangles.

  • Geometry conditions: user can add some condition to validate the geometries with a minimal segment length, a minimum angle between segment, a minimal polygon area and sliver polygons detection.

  • Topology checks: depending on geometry types, the user can choose 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 the user how get the result between modify the current layer and create a new layer.

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


Fix multiple errors

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

The default action can be changed with the last icon options Error resolution settings For some type of errors, you can change the default action between some specific action or No action.

Finally, you can choose which attribute to use when merging features by attribute value.