The layer and selected rule will be added to the list as in figure 6. Select Road layer then select the rule in the drop down list next to it. For identify undeshoot and overshoot error, the rule which applied is must not have dangles. Now come to very important step, defining rules. Topoloy Rule Settings window will appear.Ĥ. You can use the topology checker's icon or access it from menu vector as in figure 4.ģ. For this case I used road network dataset.Ģ.
If the plugin already installed, then lets see how to check topology error.Ĭhecking Undershoot and Overshoot (Dangle) Topology Errorįirstly we will check topology error for line feature.
Check the topology checker plugin and then push Install Plugin button.įigure 3. Type topology in search menu and topology checker plugin will be shown. Then plugins window will appear as in figure 3. To install a plugin, click menu Plugins > Manage and install plugins. To use this plugin, make sure the topology checker plugin already installed.
Topology Checker is a plugin in QGIS that can be used to check topology error.
Overshoot and Undershoot topology errorĬhecking hundreds or even thousands features with topological error is In figure 2 can be seen the illustration of overshoot and undershoot topology error.įigure 2. So when we zooming in to the intersection, the line is not stop at the boundary, but it passes it a little bit. Opposites to undershoot, overshoot is a topology error when a line touch another line but it passes the line with a very short length. For example a road which is represented by a line must be connected with other line at road intersection. Undershoot is a topology error that can be found in line dataset which is a line is not touching another line segment. Overlap, gap and sliver polygon topology errorĪdditional topology error is undershoot and overshoot which are grouped as dangle. In figure 1 can be seen the illustration of these topology errors.įigure 1. With polygon, then it becomes sliver polygon with a very small area. Sliver polygon happen where there is very small polygonĪlong the common boundary. Sliver polygonĪnother topology error which can be found in polygon feature is sliver polygon. It is impossible for a parcel again which is neighbor with another parcel has no data area (blank area) around their common boundary. Gap is a topology error which is opposite to overlap, where two adjacent features which share a common boundary, contains a blank area, which is no data.
For example a parcel, it is not possible some part of it, is over another parcel. This become an error, because it is impossible for a feature to have same position with other. Overlap is a topology error where a part or whole part of a feature occupies the same position with another feature. There are some errors that can be found in a dataset such as overlap, gap, sliver polygon, undershoot and overshoot.
Topology is very important to check a data free from errors. It describes a spatial connection between objects like overlap, touches, cross, intersect, within, etc. Topology defines mathematical relationship between features in geographic region.