Checking for topological errors and fixing them

Sometimes, the data that we receive from different sources or data that results from a chain of spatial processing steps can have problems. Topological errors can be particularly annoying, since they can lead to a multitude of different problems when using the data for analysis and further spatial processing. Therefore, it is important to have tools that can check data for topological errors and to know ways to fix discovered errors.

Finding errors with the Topology Checker

In QGIS, we can use the Topology Checker plugin; it is installed by default and is accessible via the Vector menu Topology Checker entry (if you cannot find the menu entry, you might have to enable the plugin in Plugin Manager). When the plugin is activated, it adds a Topology Checker Panel to the QGIS window. This panel can be used to configure and run different topology checks and will list the detected errors.

To see the Topology Checker in action, we create a temporary...