Environment and Planning B: Urban Analytics and City Science

Many commentators have suggested that the ‘natural surveillance’ of public spaces can have important social consequences such as preventing or deterring crime. In this paper we introduce a model of natural surveillance in public space which uses visibility graph analysis. The model is applied to two study areas with very different urban morphologies: a traditional street network and a modern university campus. Levels of surveillance in the traditional street network are found to be much higher than in the university campus. It is suggested that the model can be used to test for the importance of natural surveillance as a risk-reduction factor in crime prevention through environmental design.

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.

The application of information technology to landscape analysis dates back to the early work in computer-based mapping. Indeed, much of the early development of what became geographic information systems (GIS) and three-dimensional landscape simulation was undertaken by landscape architects. Mapping of viewsheds quickly became a key element of the landscape-planning process. The process was applied to determination of both view characteristics and potential visual impacts. The algorithms for viewshed analysis were incorporated into GIS products at an early stage in their evolution, but have evolved very little since despite the identification of significant potential enhancements. Extension of the simple binary mapping of GIS has therefore depended on specific developments by individual researchers. These GIS extensions have generated models of visual quality and visual impact using mapped variables. More recently it has become apparent that the essentially two-dimensional approach to view analysis afforded by GIS is inadequate in situations with strong three-dimensional elements. The upsurge in agent-based modeling has demanded a new standard in computer-based visual interpretation of landscape. Both the historic role of GIS-based visual modeling and the potential of 3D-based visual modeling are reviewed.

This paper describes the history and current state of archaeological visibility studies. The first part is a survey of both GIS (geographic information systems) and non-GIS studies of visibility by archaeologists, which demonstrates how advances in GIS visibility studies have tended to recapitulate, albeit over a compressed timescale, theoretically driven developments in non-GIS studies. The second part presents an example of the kind of methodological development required for the use of GIS to contribute to the agenda set by certain strands of a more humanistic archaeology. An algorithm developed to retrieve various summaries of the inclination at which points on the horizon are visible from a specified viewpoint was applied to nineteen recumbent stone circles in the Grampian region of Scotland. The results suggest that these summaries provide a useful tool for ‘unpacking’ what archaeologists mean when they claim that the topographic setting of certain stone circles creates an ‘impression of circularity’.

Several environment applications require the computation of visibility information on a terrain. Examples are optimal placement of observation points, line-of-sight communication, and computation of hidden as well as scenic paths. Visibility computations on a terrain may involve either one or many viewpoints, and range from visibility queries (for example, testing whether a given query point is visible), to the computation of structures that encode the visible portions of the surface. In this paper, the authors consider a number of visibility problems on terrains and present an overview of algorithms to tackle such problems on triangulated irregular networks and regular square grids.

Visibility computation was crucial for computer graphics from its very beginning. The first visibility algorithms in computer graphics aimed to determine visible surfaces in a synthesized image of a three-dimensional scene. Nowadays there are many different visibility algorithms for various visibility problems. We propose a new taxonomy of visibility problems that is based on a classification according to the problem domain. We provide a broad overview of visibility problems and algorithms in computer graphics grouped by the proposed taxonomy. We survey visible surface algorithms, visibility culling algorithms, visibility algorithms for shadow computation, global illumination, point-based and image-based rendering, and global visibility computations. Finally, we discuss common concepts of visibility algorithm design and several criteria for the classification of visibility algorithms.

In this essay we review the major developments in the history and techniques of landscape visibility analysis, providing a number of examples and identifying a few critical challenges to the community of those who would seek to evaluate visibility—and related characteristics such as visual quality or preference—in landscapes. We argue that visibility per se is a necessary prerequisite, but insufficient in itself, as a basis for the common planning, design, and public policy questions in the context of which many visibility analyses are instigated.

Mobile cellular communication has already entered the mass market, and mobile Internet services will soon become a reality. The frequent use of mobile radio technologies wherever people are has a direct impact on the deployment of base stations or radio access points, including antennas. Put simply, to serve an increasing number of users requires an increasing number of base stations. Thus, operators must carefully plan the deployment and configurations of radio base stations in order to support voice and data traffic at a level of quality expected by customers. Planning tools are used to help radio engineers in their difficult tasks of balancing requirements or radio coverage and quality with economic and other practical aspects. These planning tools make extensive use of terrain databases and of visualisation tools that lead some tool vendors to offer functionalities very similar to a geographical information system (GIS) or even to base their product on a GIS. Futhermore, because radio communication between base stations and users is crucial, all computations in a planning tool are based on the use of radio-propagation predictions. Until recently, empirical propagation prediction seemed sufficient. However, more efficient planning and the planning of nonvoice services or of a mixture of voice and nonvoice services require more accurate propagation-prediction models. These propagation models are usually based on the computation of the physical interaction of ratio waves and the environment. Thus more detailed databases are required, especially in urban environments where most users are located. Although the authors are not GIS specialists, they aim to expose some of the relationship between radio-propagation models used for mobile radio network planning and databases for terrain and buildings. For example, an accuracy of about 1 m on the coordinates of building corners is found to be acceptable in terms of accuracy in radio coverage. As another example, simulation results show that the use of conventional propagation models and rough geographical databases for the planning of future cellular systems like UMTS might cause serious difficulties.