How do I find polygons

Geometry check pluginĀ¶

Geometry checking is a powerful core plugin for checking and repairing the geometries of a layer. The Geometry review Dialog shows various grouped settings in the first tab (Settings):

  • Input vector layer: select the layer to be checked. A Selected objects only Checkbox can only filter the geometries for the selected ones.

  • Geometry validity: gives the user a choice between Self-intersection, Double knot, Polygons with less than 3 nodes.

  • Allowed geometry types: only some geometry types such as point, multipoint, line, multiline, polygon and multipolygons are possible.

  • Geometry properties shows Polygons and multipolygons cannot contain holes and Multi-part objects must consist of more than one part.

  • Geometry conditions: Users can add some conditions to confirm geometries with a minimum segment length, a minimum angle between segments, a minimum polygon area and silver polygon detection.

  • Topology check: checks duplicates, objects within other objects, overlaps smaller than a number and gaps smaller than a number.

  • tolerance: here you can set the tolerance for the check.

  • Output vector layer gives the user the choice of how he wants the output layer, as a modification of the current layer or as a new layer.

Once you are happy with the configurations, you can [Start] click.

The results appear in the second tab and as an overview layer the error in the map view (its name is tester). A table list of the Geometry check result with one error per line: the first line is an ID, the second the reason for the error, then the coordinates of the error, a value (depends on the error type) and finally the resolution column, which shows the resolution of the error. At the end of this table you can export the errors to a shape file. On the left you have the number of errors and the errors that have been fixed.

Figure Geometry Checker:

The geometry check plugin

The Geometry checking plugin can find the following errors:

  • Self-intersection: a polygon with a self-intersection,

  • Double knots: two double knots in one segment

  • Holes, holes in a polygon,

  • Segment length, a segment length, less than a threshold,

  • Minimum angle: two segments with a smaller angle than a threshold,

  • Minimum area: polygon areas less than a threshold

  • Silver polygons: this error comes from very small polygons (with a small area) with a large circumference,

  • Duplicate objects,

  • Object within object,

  • Overlaps: overlapped polygons,

  • Gaps: Gaps between polygons

The following figure shows the different checks of the plugin.

Figure The differences checks supported by the plugin

The various tests supported by the plugin

You can select a line to see the location of the error. You can change this behavior by choosing another action between error (Preset), object, Dont move, and Highlight the contour of the selected layer.

Below the zoom process, when you click on the table row, you can choose between: Show attribute table of the selected object, Fix selected errors with standard resolution other Correct selected errors, query standard resolution choose. Later you will see a window to choose the resolution method:

  • Merge neighboring polygons with the longest common edge,

  • Combine neighboring polygons with the largest areas,

  • Combine neighboring polygons with identical attribute value, if present or retained

  • Delete object

  • No action

tip

Fix several bugs

You can fix several errors if you select more than one row in the table by using Ctrl + click to press.

The default action could be changed with the last icon Troubleshooting settings. For the same types of errors, you can change the default action between some specific action or: gui label: No action choose.

Finally you can Attribute to use for object summary by attribute choose.