Isochrones and catchment areas

(outline, to be written out)

An isochrone is the area reachable from a location within a given travel time. A catchment area, or service area, is the set of locations served by a facility. Both are one to all Dijkstra results with a cut off, presented spatially, and both are the visual form in which accessibility results most often reach a non technical audience.

Planned contents

• Computation: one to all routing with a cut off, and why this is cheaper than it looks
• From reachable links to a polygon: the choice between link buffers, convex and concave hulls, and raster based approaches, and how much the resulting map depends on that choice alone
• Grid based isochrones: computing impedance to every cell instead, avoiding the polygon question entirely
• Direction matters: isochrones from a point are not isochrones to it on a directed network
• Time dependence: isochrones for public transport depend on departure time and are properly reported as a distribution over a departure window rather than a single shape, see Public transport routing
• Catchment allocation: assigning each origin to its nearest or most attractive facility with service_area, and the competition corrected alternatives in Accessibility indicators

Related

• Shortest path and the Dijkstra algorithm · Accessibility indicators