Automatic generation of the axial lines of urban environments to capture what we perceive

Abstract

Based on the concepts of isovists and medial axes, we developed a set of algorithms that can automatically generate axial lines for representing individual linearly stretched parts of open space of an urban environment. Open space is the space between buildings where people can freely move around. The generation of the axial lines has been a key aspect of space syntax research, conventionally relying on hand-drawn axial lines of an urban environment, often called axial map, for urban morphological analysis. Although various attempts have been made towards an automatic solution, few of them can produce the axial map that consists of the least number of longest visibility lines, and none of them really works for different urban environments. Our algorithms provide a better solution than existing ones. Throughout this article, we have also argued and demonstrated that the axial lines constitute a true skeleton, superior to medial axes, in capturing what we perceive about the urban environment.

Keywords:

• visibility
• space syntax
• topological analysis
• medial axes
• axial lines
• isovists

Acknowledgements

Part of the research was carried out while the authors worked with the Hong Kong Polytechnic University. Thanks to Alasdair Turner for kindly sharing the data sets of Gassin, Barnsbury and National Gallery London, and to the referees for their constructive comments. We also thank Junjun Yin for his assistance in digitizing the maps of the street patterns and Petra Norlund for polishing up our English.