By David Crowther
Question:
How can I calculate isochrones from a set of start points using 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 Layer
In this QGIS project we have a line network from the Ordnance Survey – ITN_TF and a series of start locations, such Council Tax Payment Centres, represented by the Purple Circles.
Using the Service Area – from Layer tool we will generate Service Areas (represented as a spider’s web) from each of the start locations, using the road network travelling a specified distance.
In QGIS choose Processing > Toolbox > and open the Network Analysis section.
Choose the Service Area > from Layer 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).
- Vector Layer with Start Points – choose the Layer of Points to calculate the distance from – Council Tax Payment Centres.
- 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 1750 metres along the road network.
Press Run and the calculation from the starting points along the road network, travelling 1750 metres will be undertaken.
The output will be a new Line Layer, with line segments from the start locations (Council Tax Payment Centres) up to 1750 metres along each road network, creating a spider’s web from each starting point.
The result is a new line layer, representing a 1750 metres distance along that line network from each starting 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 a number of isochrone (or in this case isodistance) polygons, which you can then use to do point in polygon spatial analysis against to determine for example if all the address points in your local authority are within a 1750 metre distance of a Council Tax Payment Centre.
To create the Isodistance Polygons, we will use another QGIS tool from the Processing Toolbox – Convex Hull.
- Choose the Input Layer to be the newly created Service Areas Line layer.
- Leave all other settings as they are.
Press Run and Convex Hull polygons are created using the end vertices of the new surface layer line network to create closed polygons around each of the spider’s web line networks.
Once added to the map, the new Isodistance polygons can then be used to spatially query which address points fall within 1750 metres, using the road network, from the Council Tax Payment Centre locations.
This then enables you to identify the population that are not being served and can help you target locations for additional services. Alternately you can now see areas of overlap, which can help you choose to move existing services to more optimal locations.