Creating a simple routing network

In this recipe, we will create a routing network from scratch using the QGIS editing tools. Even though more and more open network data is available, there will still be numerous use cases where necessary network data does not exist or is not available for use. Therefore, it is good to know how to create a network and what to pay attention to in order to avoid common pitfalls.

For the task of network creation, the main difference between the Road graph plugin and pgRouting is that pgRouting needs a network node (that is, link start or end node) at each intersection while the Road graph plugin will also use intermediate link geometry nodes to infer intersections if two links share a node. In this recipe, we will create a network, which can be used in both tools.

Getting ready

To follow this recipe, you only need a new empty QGIS project. Additionally, make sure you have the Digitizing toolbar enabled (as shown in the following screenshot). We will create an imaginary network, but if you want you can load a background map and digitize this:

How to do it…

Before we can start to create the network, there are a few things that need to be set up first:

Create a new shapefile line layer for the network. You don't need to add any extra attributes besides the default ID attribute yet.
Note
You can read more about creating new shapefiles in the Learning QGIS book by Packt Publishing and the QGIS user guide at http://docs.qgis.org/2.2/en/docs/user_manual/working_with_vector/editing_geometry_attributes.html#creating-a-new-shapefile-layer.
To ensure that we can digitize the network with valid topology, we'll activate snapping next. Go to Settings | Snapping Options and activate snapping for your line layer by enabling the checkbox to the left of it. Additionally, set the mode to to vertex and choose a tolerance of at least 5.00000 pixels:
Now, we can enable editing for the line layer and then select the Add Feature tool from the Digitizing toolbar to start digitizing.
Create the first line feature now, and give it the ID number 1. The line can have as many nodes as you wish. We'll create a line with four nodes, as shown in the following screenshot:
To draw the second line feature, start at the first or the last node of line 1. As we have activated snapping, you will see that the node is being highlighted if you hover over it with the mouse cursor. Draw a second line and give it the ID number 2.
Note
The line in the preceding screenshot is drawn with a style that has circles on the starting and ending points. You can reproduce this style by adding the Marker line symbol levels to the line style or load network_links.qml from our sample data. For more details about styling features, please refer to Chapter 10, Cartography Tips.
Draw a few more lines (around 12 in total) forming a network. Make sure to pay attention to the snapping and assign link IDs:
Disable editing, and confirm that you want to save the changes.

We will use this basic network as a starting point for the remaining recipes in this chapter.

How it works…

By setting the snapping mode to to vertex, we made it possible to digitize the line network in a way that ensures that lines, which should be connected, really contain a node at the exact same position.

There's more…

You can validate the network topology by running the Topology Checker plugin, which is installed with QGIS by default (you can read more about Topology Checker in Chapter 12, Up and Coming):

Start Topology Checker from the Vector menu.
Click Configure to set up a topology rule, as shown in the following screenshot, and click on Add Rule to add it to the list of rules to check. Then, close the settings by clicking on OK:
Once this tool is configured, click on Validate All (the button with the checkmark) to initiate the check. You will see the list of discovered errors displayed in the list above the buttons, as shown in the following screenshot. Additionally, the dangling ends are highlighted in red in the map:
You can select the error entries in the list to jump to the line features that failed the check. In our network, only lines with dead-ends should be listed. If you see an error at an intersection, you should zoom closer and try to correct the node positions.

Previous Chapter

6. Network Analysis

Next Chapter

Calculating the shortest paths using the Road graph plugin

Table of Contents for QGIS: Becoming a GIS Power User