In network analysis with QGIS, a service area refers to a geographic zone that is covered or served by a specific transportation or distribution network. This can include elements such as roads, public transit lines, water or electricity networks, telecommunications infrastructures, etc.
The service area represents the geographical extent from which users can access the services or resources available via the network. It can be defined according to various criteria, such as maximum distance, travel time, network capacity, etc.
Service area (of a point)
If you open the Service area (point) tool, you’ll find a settings window similar to those for the shortest path.
The difference is in the Travel cost field. In this field, we’ll enter
- either the maximum distance of our service area, from the departure point, in map units,
- or the maximum travel time from the starting point, in hours.
Service area: the simplest case
If we leave the default options, the tool calculates the service area (in mauve) by calculating all possible routes, with a length of 2 Km from our starting point.
Service area: traffic directions taken into account
These theoretical distances do not take into account prohibited directions. For this, we’ll do as we did for the shortest routes, by configuring a direction field. To take traffic directions into account, we use the oneway attribute in OSM. A value of B indicates that the section is two-way, and a value of F indicates that the direction of traffic is the direction in which the section is digitized. A value of T indicates that the section cannot be used.
The result (in yellow) shows that the catchment area is smaller than calculated.
But in this urban area, most roads have a speed limit of 45 km/h, while others are limited to 60 Km/h.
At 45 Km/h our 2 Km are covered in 2 minutes 40 seconds. If the limiting factor in our service area is time, we can modify the parameters to find all the routes that can be covered in 2m40s (0.044333 hours). To do this, we set the speed field to use OSM’s maxspeed attribute and change the type of trip to be calculated to The fastest.
The result is a dessert area (in red) that is larger than the one calculated using only the traffic directions, because using the faster routes means you can travel further while keeping the same journey time.
The other available tool, Service area (of the layer), performs the same calculations but for each point contained in the layer. The result is a service area per point.