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6. Topology

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Hi my name is Marcelle. Welcome to the 6th topic in our gentle introduction to GIS series. In this screencast, we will give you a basic introduction to the concept of topology. We will show you how to use one of the topological editing tools in QGIS. Topology is about spatial relationships. For example, the relationship between connecting ... ... or adjacent vector feature in a layout. Topological tools are useful for detecting and correcting digitising errors. Topological data can also be used for some special types of special analysis. For example, to find the shortest route between two points ... ... on a road system we would use a network analysis. There are different types of topological errors: overshoots, undershoots and slivers... ... or all topological errors that we discuss in topic 2. Topological errors, in general, break the relationship between vector features. These relationships need to be fixed to get accurate analysis results. To prevent the user from making topology errors during digitising, ... ... GIS applications often provide topological editing tools. These tools make sure that certain topological rules are enforced. They help the user to edit a vector layer in a topologically correct way. Let's show you how topological editing works with QGIS. In this example, we will show you how to ensure that ... ... we don't create polyline overshoots and undershoots. We will digitise additional polyline road features in a existing road layer. The topological tool used for this is called Snapping. The snapping distance is the distance a GIS uses to search for the closest vertex ... ... and/or segment that you are trying to connect to when you digitise. A segment is the section of line between two vertices. First, we need to load a roads vector layer. To do this, click on the 'Add Vector Layer' icon in the toolbar Navigate to your local data directory under: ... ... C:... ... Program Files ... ... Quantum GIS ... ... Gisdata ... ... and then in 'local'. You should see a list of vector layers. Choose the layer called '50k_roads' and then click 'open'. We are going to be adding some new (make believe) roads to our road network. Before we start digitising we need to enable snapping. To do this, first click on the 'Settings' menu. Now click on the 'Project Properties' option. Activate the 'enable topological editing' checkbox and click ok. Now click on the 'Settings' menu again. Next choose 'Options' Click on the 'digitising' tab. Set the 'default snap mode' to vertex and segment. This will ensure that when we create a new road close to an existing one, ... ... our new road will connect to the nearest part of the existing road. Change the option 'default snapping distance in layer units' to 0.0005. This represents a distance of around 50 m. Change the 'option default search radius' to 0.005. 0.005 in decimal degrees is about 500 meters. Now click 'OK' to save your changes. The topological tools needed for snapping are now activated ... ...and we can start digitising! Just like we did in our screencast on data capture, ... ... we enable the editing mode of the roads layer. To do this, click on the toggle editing icon on the toolbar. Now, click the 'zoom in' tool. Let's zoom to an interesting area with many roads. We are now ready to add some more roads. Click on the 'Capture line' Icon in the toolbar, to activate polyline digitising mode. Now start digitising by clicking on the left mouse button ... ... close to an existing vertex or segment. Because snapping mode is active, ... ... we will automatically be connected to the closest segment or vertex. Now, if we move the mouse around, we will see, ... ... that QGIS tries to connect our new node to the closest vertex or segment. It only does this if there is another road close by. Once you have finished digitising your new road, ... ... right click to finalise the line. In the dialog that appears, type in capital letters, ... ... SECONDARY ROAD into the 'feature type' box. Then click 'OK' to save the new road. Once we have digitized some new roads, we can stop ... ... this sample session by clicking on the 'toggle editing' icon. Remember to choose save when QGIS asks you... ... if you want to keep your changes. We can see that as well as ensuring that our roads are all connected, ... ...the snapping topological tool makes it much easier and faster to capture new lines. Well, that brings us to the end of this screencast. See you next time!

Video Details

Duration: 11 minutes and 50 seconds
Country: South Africa
Language: English
Producer: Chief Directorate: Spatial Planning & Information, Department of Land Affairs, Eastern Cape, South Africa
Views: 869
Posted by: giacomo on Mar 30, 2010

Topology defines the relationship between vector features. In this topic we look at how topology is used in a GIS, and the differences between topological and non topological datasets. We also look at how topological concepts can be used to improve the digitising process.

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