The concept of an isovist has had a long history in both architecture and geography, as well as mathematics. Tandy (1967) appears to have been the originator of the term isovist'. He presents isovists as a method of``taking away from the [architectural or landscape] site a permanent record of what would otherwise be dependent on either memory or upon an unwieldy number of annotated photographs'' (page 9). The same idea has a similarly long history in the guise of the`viewshed' in the field of landscape architecture and planning (Amidon and Elsner, 1968; Lynch, 1976) and in terms of intervisibility' in computer topographic models (Gallagher, 1972). The appeal of the concept is that isovists are an intuitively attractive way of thinking about a spatial environment, because they provide a description of the spacè from inside', from the point of view of individuals, as they perceive it, interact with it, and move through it. As such, isovists have particular relevance to architectural analysis. Benedikt (1979) introduced a set of analytic measurements of isovist properties to be applied to achieve quantitative descriptions of spatial environments. Benedikt starts by considering the volume visible from a location and then simplifies this representation by taking a horizontal slice through the`isovist polyhedron'. The resulting`isovists' are always single polygons without holes, as shown in figure 1 (see over). Consequently, Benedikt considers geometric properties of isovists, such as area and perimeter. Thus he begins to quantify space, or what our perception of space might be, and the potential for its use. Benedikt notes that, in order to quantify a whole configuration, more than a single isovist is required and he suggests that the way in which we experience a space, and how we use it, is related to the interplay of isovists. This leads

Axial analysis is one of the fundamental components of space syntax. The space syntax community has suggested 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. Why not another representation? In particular, why not road-centre lines, which are easily available in many countries for use within geographical information systems?Here we propose that a recently introduced method of analysis, angular segment analysis, can marry axial and road-centre line representations, and in doing so reflect a cognitive model of how route choice decisions may be made. We show that angular segment analysis can be applied generally to road-centre line segments or axial segments, through a simple length-weighted normalisation procedure that makes values between the two maps comparable. We make comparative quantitative assessments for a real urban system, not just investigating angular analysis between axial and road-centre line networks, but also including more intuitive measures based on block-distances between locations.We show that the new angular segment analysis algorithm produces better correlation with observed vehicular flow than both standard axial analysis and block-distance measures. The results imply that there is no reason why space syntax inspired measures cannot be combined with transportation network analysis representations in order to create a new, cognitively coherent, model of movement in the city.

IntroductionIn the literature on cognitive psychology, the issues raised by Gibson's ecological theory of perception (Gibson, 1979) have been examined and taken on board. Gibson's theory was formulated primarily in order to overturn theories laden with subjective and objective knowledge, and to replace them with a model in which the agent and its environment are conjoined by a set of affordances so the agent perceives the contents of the environment directly and uses the affordances within it to guide its action without reference to superior representational models. Today, Gibson's work has been contextualised and broken into further models in which recognition and representation do play a part (Neisser, 1994). However, in the domain of agent-based modelling we still appear to ignore the original concerns he voiced öin particular`the model' is always preceded by a theoretical framework, rather than simply being a perceptual model in its own right (see, for example, Casti, 1998;Epstein and Axtell, 1996). This paper is not an attempt to overturn the body of literature which already exists in agentbased modelling, although it does constitute a plea for the use of direct perception where the approach is available, and to try to regard the environment as the provider of possibilities rather than as a place to be rationalised. As an example, consider human movement around an art gallery. There might be any number of causal factors for the routes people take. People might, for example, follow a map, or signage, take into account the direction other people are taking, a glimpse of a familiar painting, reject a route on the grounds of personal prejudice against a style, and so on. On the other hand, the possibility of exploring the walkable surface of the layout ahead (the rooms of the gallery) may simply be enough for a human to do exactly that. Abstract. Gibson's ecological theory of perception has received considerable attention within psychology literature, as well as in computer vision and robotics. However, few have applied Gibson's approach to agent-based models of human movement, because the ecological theory requires that individuals have a vision-based mental model of the world, and for large numbers of agents this becomes extremely expensive computationally. Thus, within current pedestrian models, path evaluation is based on calibration from observed data or on sophisticated but deterministic route-choice mechanisms;there is little open-ended behavioural modelling of human-movement patterns. One solution which allows individuals rapid concurrent access to the visual information within an environment is an`exosomatic visual architecture', where the connections between mutually visible locations within a configuration are prestored in a lookup table. Here we demonstrate that, with the aid of an exosomatic visual architecture, it is possible to develop behavioural models in which movement rules originating from Gibson's principle of affordance are utilised. We apply large numbers of agents programmed with these r...

The fewest-line axial map, often simply referred to as the ‘axial map’, is one of the primary tools of space syntax. Its natural language definition has allowed researchers to draw consistent maps that present a concise description of architectural space; it has been established that graph measures obtained from the map are useful for the analysis of pedestrian movement patterns and activities related to such movement: for example, the location of services or of crime. However, the definition has proved difficult to translate into formal language by mathematicians and algorithmic implementers alike. This has meant that space syntax has been criticised for a lack of rigour in the definition of one of its fundamental representations. Here we clarify the original definition of the fewest-line axial map and show that it can be implemented algorithmically. We show that the original definition leads to maps similar to those currently drawn by hand, and we demonstrate that the differences between the two may be accounted for in terms of the detail of the algorithm used. We propose that the analytical power of the axial map in empirical studies derives from the efficient representation of key properties of the spatial configuration that it captures.

Recently there has been a revival of interest in visibility analysis of architectural configurations. The new analyses rely heavily on computing power and statistical analysis, two factors which, according to the postpositivist school of geography, should immediately cause us to be wary. The danger, they would suggest, is in the application of a reductionist formal mathematical description in order to ‘explain’ multilayered sociospatial phenomena. The author presents an attempt to rationalise how we can use visibility analysis to explore architecture in this multilayered context by considering the dynamics that lead to the visual experience. In particular, it is recommended that we assess the visual process of inhabitation, rather than assess the visibility in vacuo. In order to investigate the possibilities and limitations of the methodology, an urban environment is analysed by means of an agent-based model of visual actors within the configuration. The results obtained from the model are compared with actual pedestrian movement and other analytic measurements of the area: the agents correlate well both with human movement patterns and with configurational relationship as analysed by space-syntax methods. The application of both methods in combination improves on the correlation with observed movement of either, which in turn implies that an understanding of both the process of inhabitation and the principles of configuration may play a crucial role in determining the social usage of space.

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