Automatic Generation of the Axial Lines of Urban Environments to Capture What We Perceive

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 paper, 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. 1. Introduction Open space of an urban environment or of a city is the space where people can freely move around, and it is in contrast to closed spaces such as buildings and street blocks. Open space can be thought of as something between closed spaces, and can be represented as a complex polygon with holes. The holes denote closed spaces, i.e., the buildings and street blocks. Open space of a city forms one continuous piece rather than separate parts, since people can travel from anywhere to anywhere else in a city. For the purpose of modeling human movement, open space has been of primary interest, being the modeling target in space syntax (Hillier and Hanson 1984). It is done through representing open space as many perceivable units, and further forming a connectivity graph consisting of nodes representing individual units and links if the units are intersected. The connectivity graph can be used to analyze and rank the status of the individual units for understanding the underlying topology of the city. The representation of open space can be in different ways.