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Measuring Accessibility Using GIS

JULIAO, Rui Pedro (rpj@fcsh.unl.pt) Departamento de Geografia e Planeamento Regional, Universidade Nova de Lisboa/FCSH, Av. Berna, 26 C, 1050 Lisboa, Portugal

Keywords: accessibility, planning, regional development

Today it is widely understood that one cannot promote development regardless of different territorial specifications that make the nation mosaic. We cannot promote the development of the whole without the knowledge of each small territory and the relationship between them. This is why recent territorial planning is facing these principles. Planning policies are concerned with equity and a better distribution of people and activities in the territory, which is why accessibility, regardless if it is measured in time, cost, distance, or population, are the most important variables that one must consider in the early stages of planning. In what concerns population studies there are several different methodologies for its analysis and understanding, considering spatial interaction. Unfortunately, accessibility is widely evaluated by methodologies that do not consider a real spatial model. The traditional methods for accessibility evaluation do not consider the whole territory; they are mainly based in matrix methods and in node/arc logic. By using this type of method, one cannot get information to the whole territory. It was usual to get an accessibility index for a specific node of the network, but not for any point. Today accessibility measurement, regardless if its unit is time, cost, or distance, must be evaluated for the whole territory and not only on the network.

The usual methods for accessibility evaluation, based in graph theory, are quite easy to essay in a vector format GIS analysis, but if one wants to create a continuous model, we must work in a raster environment. This, of course, will reduce the geometrical accuracy of the information; however, it opens a wide range of new analysis capabilities. This is why although the original information was in vector format, the analysis was made mainly using raster data. The general structure of the accessibility evaluation methodology has 3 phases:

- Data Acquisition and Integration

- Cost Surface Modelling

- Accessibility Analysis

This paper describes the development and application of a GIS-based methodology for accessibility evaluation, and its different potential applications in planning studies using the Lisbon and Tagus Valley Region as a case study area.