Could A Road-centre Line Be An Axial Line In Disguise?

Axial analysis is one of the fundamentals of space syntax. Hillier has proposed that it picks up qualities of configurational relationships between spaces not illuminated by other representations. However, critics have questioned the absolute necessity of axial lines to space syntax, as well as the exact definition of axial lines. An often asked question is: why not another representation? In particular, why not road-centre lines, which are easily available in many countries for use within geographical information systems (GIS)? The major difference between road-centre line networks and axial networks is that road-centre lines are broken across junctions, and therefore graph measures of the corresponding road-centre networks tend to vary with physical distance rather than the changes of direction as measured within axial networks. As a solution it is has been proposed that we abandon both changes of direction and physical distance as graph measures: instead we should use the angular change between segments. There are strong theoretical and cognitive arguments for this new approach; however, to date, no quantitative studies have been undertaken. Here we show how angular analysis may be applied generally to road-centre line segments or axial segments, through a simple normalisation procedure that makes values between the two maps comparable. This allows us to make comparative quantitative assessments for a real urban system. As a test case, we take the Barnsbury area of north London, and show that the new algorithm produces good if not better correlation with vehicular flow than standard axial analysis. Furthermore, both our theory and results show that a normalised version of the graph measure of betweenness (or “choice” as it is known within space syntax) is a better model of movement than integration; this further implies there is a strong case for a review of how we use axial analysis in practice.