Topology Error

Figure 1. Overlap, gap and sliver polygon topology error

Figure 2. Overshoot and Undershoot topology error

QGIS Topology Checker Plugin

Figure 3. Installing Topology Checker Plugin

Checking Undershoot and Overshoot (Dangle) Topology Error

Figure 4. Topology Checker Menu

Figure 5. Topology Checker Panel

Figure 6. Topology Rule Settings

Figure 7. Topology errors are marked with red marker

Figure 8. Dangle Error (Undershoot)

Checking Polygon Topology Error

Topology defines mathematical relationship between features in geographic region. It describes a spatial connection between objects like overlap, touches, cross, intersect, within, etc. Topology is very important to check a data free from errors. There are some errors that can be found in a dataset such as overlap, gap, sliver polygon, undershoot and overshoot.Overlap is a topology error where a part or whole part of a feature occupies the same position with another feature. This become an error, because it is impossible for a feature to have same position with other. For example a parcel, it is not possible some part of it, is over another parcel.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. It is impossible for a parcel again which is neighbor with another parcel has no data area (blank area) around their common boundary.Another topology error which can be found in polygon feature is sliver polygon. Sliver polygon happen where there is very small polygon along the common boundary. Or we could say when a gap is filled with polygon, then it becomes sliver polygon with a very small area. In figure 1 can be seen the illustration of these topology errors.Additional topology error is undershoot and overshoot which are grouped as dangle.Undershoot is a topology error that can be found in line dataset which is a line is not touching another line segment. For example a road which is represented by a line must be connected with other line at road intersection.Opposites to undershoot, overshoot is a topology error when a line touch another line but it passes the line with a very short length. So when we zooming in to the intersection, the line is not stop at the boundary, but it passes it a little bit. In figure 2 can be seen the illustration of overshoot and undershoot topology error.Checking hundreds or even thousands features with topological error is tiring and time consuming. Because of that, we need a tool that can help us to check topology error. Topology Checker is a plugin in QGIS that can be used to check topology error. To use this plugin, make sure the topology checker plugin already installed. To install a plugin, click menu Plugins >> Manage and install plugins.. Then plugins window will appear as in figure 3. Typein search menu and topology checker plugin will be shown. Check the topology checker plugin and then push Install Plugin button.If the plugin already installed, then lets see how to check topology error.Firstly we will check topology error for line feature. For this case I used road network dataset.1. Add the road data to QGIS.2. Open topology checker. You can use the topology checker's icon or access it from menu vector as in figure 4.3. Topology checker panel will appear. Clik configure button as in figure 5. Topoloy Rule Settings window will appear.4. Now come to very important step, defining rules. For identify undeshoot and overshoot error, the rule which applied is. Select Road layer then select the rule in the drop down list next to it. When finish select Add Rule Button. The layer and selected rule will be added to the list as in figure 6. Then click OK button.5. After defining rule, in topology checker panel click button Validate All or Validate Extent. Validate All will check the topology error for the defined rule all over the data. On the other hand Validate Extent will check error in the extent of map view. If you have a big number of features (thousands) it will be better to validate in extent to speed up the process. The error will be shown in topology checker panel list and also will be marked on the map view with red marker as in figure 7.6. Zoom in to the area with red marker to see the error clearly. You will see the error as in figure 8.We have checked the error for line feature. Now lets move to polygon feature. We will check for topology error such as overlap and gap.