How are the catchment areas (drive, walk, cycle) generated and which datasets are used?
Active Places Power uses ESRI's ArcGIS Server Network Analyst software to provide dynamic catchment area analysis as part of the Catchment Report and Area Access Report.
Three network analysis models of Drive, Walk and Cycle are available, which use a combination of Ordnance Survey's OS Highways (previously Integrated Transport Network ITN) Layer, Ordnance Survey's Paths data and Sustran’s National Cycle Network
The ArcGIS Server Network Analyst extension computes catchment areas (isochrones) around any location on the network whether this be an existing facility or a new user defined location. The service area will encompass all accessible streets that are within a specified impedance. For example, the 5-minute catchment area for a point on a network includes all the streets that can be reached within five minutes from that point. A service are for a given point is simply the polygon areas defined by specific time limit breaks (i.e. 5, 10, 15, 20 , 25, 30, and 35 minutes).
The resulting isochrones are then intersected with profile data (point in polygon) to create the required report information. For example, population centroids within each catchment area are selected and aggregated to provide a total population count.
For details on how access analysis outputs are generated please see the FAQ: How does the access analysis report determine accessibility of sport facilities for a defined population?
Update: May 2020
As of 6th May 2020, Ordnance Survey Highways data is used in the Active Places Power web site. This dataset is more refined than ITN, with considerable additions and updates of roads and paths in the UK.
When users execute the Catchment and Area Access functions, the results they see may be different from previous model runs. The difference is manifested by the fact that the OS Highways dataset is a new schema and geometry refinement of the previous legacy ITN dataset. The Active Places transport network analysis model, which uses OS Highways to create the dynamic routing solutions, is now also built by the new Esri UK Data Loader module. It has a more precise tolerance level and introduces the Z coordinate capabilities for traditionally X and Y based coordinate storage for the polylines representing roads. Turn restrictions are also consolidated.
A typical catchment analysis involves trimming the outer edge polygon to provide a more cartographically simplified view, a typical tolerance of 100 metres was common when using ITN to avoid spikes and jagged edges. The same trimming level in the OS Highways provides a more refined and precise relationship to roads, as seen in Figure 1 below. Trimming level tolerance calculation is largely based on the precision, bounding extent and vertex density of the underlying geometry. Due to the fact that the OS Highways dataset is bigger and has more precision and a more refined coordinate storage the 100-metre tolerance does not look like that of the ITN legacy model.
Comparison outputs between ITN (left image) and OS Highways (right image) are provided in the below links:
Road catchment output
Path catchment output
For more information on the Ordnance Survey data products see:
London Cycling Design Standard - http://content.tfl.gov.uk/lcds-chapter6-signsandmarkings.pdf p15-16
For more information on ESRI's Network Analyst see: