Out of reach? Introducing our distance and travel time plugin
The Reachability plugin enables users to draw areas of equal distance or travel time from a given location. This can be useful for:
- determining how many pharmacies are within a 10 minute walk of a GP
- comparing public transport accessibility with employment opportunities
- drawing school catchment areas
Knowledge of the distance between locations and the time it takes to get from one location to another is particularly important for those responsible for service delivery: a person’s distance or travel time to a key service might affect their ability to access it.
Buffering is a technique available in geographic information systems (GIS) that creates one or more polygons that encloses an area within a specified distance of a spatial feature (point, line or polygon). The technique is commonly used by urban planners, ecologists and public health practitioners amongst others to measure the proximity of spatial features or events. Example uses include measuring the transmission range of mobile communication masts, calculating spatial concentrations of crime, and determining the catchment areas of retail stores.
There are two common types of buffer: euclidean and network. Euclidean buffers visualise the area that is within a specified distance whilst network buffers represent the area covered by roads, railway lines, paths (dependant on the mode of travel) within the distance specified. Buffers created using Euclidean or straight-line distance assume that space is continuous, homogeneous and uniform in all directions. In other words, Euclidean buffers assume that spatial features or events can happen anywhere in the area covered by the buffer.
Euclidean distance is ideal for an ecologist measuring the dispersal of wind-borne seeds which can land anywhere but many spatial features and events are constrained by a one-dimensional subset of planar space: network space. Consider how buildings can impair the strength of mobile signals, land use influences the spatial distribution of crime, and retail customers travel by road and rail to go shopping. The location of these features or events and often movement between them are constrained by network space.
The plot below illustrates the difference between euclidean and network buffers. It shows a 10km buffer around a location on the Øresund Bridge which links Denmark and Sweden. The Euclidean buffer ignores the path of the bridge whilst the network buffer follows the bridge and the direction of traffic. The network buffer is clearly more appropriate than the euclidean buffer for capturing network constrained features and events within 10km of the location such as road traffic collision, bridge repairs etc.
Network buffers typically come in two flavours: travel distance and travel time. Travel distance buffers use shortest path distance to measure the area reachable along a road network and travel time buffers or ‘isochrones’ represent how long it takes to travel a certain distance on foot, by car, or on public transport along that network.
Isochrones have been around since the nineteenth century and were drawn by hand. Francis Galton’s ‘Isochronic Passage Chart’ of 1881 was the first isochrone map and shows travel time from London in days. Manchester’s Tramway Department’s later ‘Time Zone Map’ of 1914 shows journey times into the city centre by tram.
Nowadays, isochrones and travel distance buffers can be created in a GIS using tools like ESRI’s Network Analyst extension or the open source pgRouting. Several route service platforms also offer web applications and APIs that create travel time polygons (e.g. Google’s Distance Matrix API and igeolise’s TravelTime.
The Lab’s isochrone plugin uses the OpenRouteService platform, an open source routing tool developed by the Heidelberg Institute for Geoinformation Technology (HeiGIT) which uses the OSM street network.
You can see the plugin in action on our Explore app: https://www.trafforddatalab.io/maps/explore/