Accessibility is the ease with which relevant destinations can be reached from a given location. It is a driving factor in land use modelling, a policy indicator in its own right, and an input to a wide range of social, economic and environmental analyses.

This wiki is a method reference. It describes how accessibility is modelled: the concepts, the modelling choices, the indicators and the pitfalls, independently of any single project.

Examples in this wiki use the GeoDMS where a concrete implementation helps, but the methods themselves are software independent.

• Accessibility, what accessibility is, its four components and four measurement perspectives
• Travel impedance, cost, time and distance as the input to every accessibility measure
• Euclidean versus network distance, why "as the crow flies" is rarely good enough

• Network data sources, choosing between OSM, NWB, GTFS and other sources
  • OSM · NWB · GTFS
• Building a routable network, from geometry to a connected, directed graph
• Network optimisation algorithm, reducing a network without changing its routing behaviour
• Travel speeds per mode, car, bicycle, pedestrian and public transport

• Zoning and spatial units, points, zones, grids, and the errors each introduces
• Connecting origins and destinations to the network, connectors, snapping and access time

• Shortest path and the Dijkstra algorithm, the routing core
• Origin Destination matrix (trip table), many origins to many destinations
• Isochrones and catchment areas, reachable areas and service areas
• Public transport routing, timetables as time expanded graphs

• Accessibility indicators, the indicator families side by side, and when to use which
• Distance decay functions, translating impedance into travel potential
• Spatial Interaction Models, the theory behind gravity and doubly constrained models
• Network centrality, centrality from random walker distributions

• Applications, projects, published indicators and downloadable datasets

• Glossary
• References