Geometric accessibility and geographic information: extending desktop GIS to space syntax

Accessibility indices which measure the relative `nearness' or `propinquity' of geographic locations, one to another, are not well-developed functions in proprietary geographic information systems (GIS), notwithstanding their evident usefulness for urban analysis. In this paper, we develop procedures for embedding such measures within desktop GIS, focusing on the measurement of geometric accessibility which is applicable to ®ne-scale urban structure at the street-building level of representation. The particular type of accessibility that we examine is based on the theory of space syntax, although our presentation generalises to many other types, and the way we embed measures of space syntax within GIS is applicable to a much wider class of measures. We ®rst review the general class of geographic and geometric accessibility measures, de®ning those in space syntax as special cases. We then outline our implementation of these through a software extension within the desktop GIS ArcView, showing how the analyst can input street systems within the software through new drawing tools, generate accessibility measures through new computational functions, and present these visually using standard ArcView outputs. The extension we propose enables accessibility measures to be integrated with the more conventional functionality of GIS analysis which is embodied in the basic software and its extensions. We illustrate the analysis using a standard space syntax example based on the street pattern of a small French town. # 1999 Published by Elsevier Science Ltd. All rights reserved.