QGIS – Network Analysis: Service Areas

By David Crowther

Question:

How can I calculate isochrones within QGIS?

Answer:

In this previous blog we explore a number of QGIS tools and plugins which allow you to perform routing analysis, including Shortest Path, Distance to Lines and Isochrone Catchment analysis  - https://www.cadlinecommunity.co.uk/hc/en-us/articles/360001338178-QGIS-Routing-and-Distance-Tools

As a follow up to that Blog, we will now look at another option in the Network Analysis toolbox – Service Area – from Point

In this QGIS project we have a line network from the Ordnance Survey – ITN_TF and a start location, such as a School, represented by the Red Triangle.

Using the Service Area – from Point tool we will generate a Service Area (represented as a spider’s web) from the chosen start location, using the road network travelling a specified distance.

In QGIS choose Processing > Toolbox > and open the Network Analysis section.

Choose the Service Area > from Point menu option and complete the menu below.

• Vector layer representing network – choose your road network.
• Path to Calculate – Shortest or Fastest (you can use Fastest if you have a road speed in your road network).
• Start Point – click in the map canvas to choose the start location.
• Travel Cost – type a value for the Distance to travel (if using Shortest) or Time to travel (if using Fastest). In this example we will travel a distance of 2000 metres along the road network.

Press Run and the calculation from the start point along the road network, travelling 2000 metres will be undertaken.

The output will be a new Line Layer, with line segments from the start location (School Address) up to 2000 metres along each road network, creating a spider’s web from the starting point.

The result is a new line layer, representing a 2000 metres distance along that line network form the chosen start location. If you used Fastest as the Path to Calculate (and you had a road speed against your road network), then the output would be the lines that can be reached in a specified time.

We can use the output line network to then generate an isochrone (or in this case isodistance) polygon, which you can then use to do point in polygon spatial analysis against to determine for example which Pupil addresses fall within the given network.

To create the Isodistance Polygon, we will use another QGIS tool from the Processing Toolbox – Convert Hull (k-nearest neighbour).

• Choose the Input Layer to be the newly created Service Areas Line layer.
• Leave all other settings as they are.

Press Run and a Convex Hull polygon is created that uses the end vertices of the line network to create a closed polygon around the spider’s web line network.

Once added to the map, the new Isodistance polygon can then be used to spatially query which points fall within 2000 metres, using the road network, from the school location.