Using desktop GIS for the investigation of accessibility by public transport: an isochrone approach

The application of existing desktop geographical information systems (GIS) to the assessment of accessibility by public transport has been investigate d. Two approaches to the measurement of accessibility-aggregate accessibili ty measures and the space-time geography framework-are described. It is sug gested that isochrones (lines of equal travel time) are a natural way to co mbine these approaches in a GIS setting. A desktop GIS application has been developed which can automatically generate isochrones for travel by public transport. The application successfully copes with the complexity of the p ublic transport of a large city (Glasgow), comprising bus, rail, and underg round services. Journeys may include an initial walk to a public transport stop, several interchanges between services of the same or different modes, and a final walk. Options provide for plotting isochrones for journeys by bus only or train only; and for exact or approximate isochrones, the latter generated more quickly. The application is described in some detail, and a ssessed. Sample outputs are presented, including an example involving furth er processing of isochrones to produce a constrained accessibility map? whi ch demonstrates the generality of this approach.