An analytical model to assess the visibility of landmarks.

Publication: Geographical Analysis

Publication Date: 01-JUL-05

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Landmarks in urban areas, such as castles, churches, bell towers, tall buildings, large hills, and tall trees, are common points of geographical reference in landscapes. However, modern construction methods have pushed buildings higher, so the visibility of these landmarks has been reduced. This article develops a simple analytical model for use in an urban setting to estimate how building height and density affect the area from where a landmark is visible. The probability of visibility from observation points will be derived by the use of a stochastic process. Three relationships between the visibility of a landmark and its setting are mathematically derived and illustrated using a monocentric city.

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Introduction

Landmarks in urban area, such as castles, churches, bell towers, tall buildings, large hills, and tall trees provide points of geographic reference in a landscape. Typically, they can be seen from many angles and distances, for example, from the top of small buildings. They can be identified according to differences in size, form, color, texture, function, symbolic meaning, and how they relate to their surroundings. Research in both psychology and geography has demonstrated that landmarks aid travel. In addition, there is a tendency for those who are more familiar with an urban area to increasingly rely on landmarks as their point of reference. Landmarks are a significant feature in a community or regional landscape, as discussed by Lynch (1960).

Modern construction methods have permitted tall buildings to be built in urban areas. Architects in Japan have tended to ignore the visual impact of new buildings on existing landmarks. This is because building size is usually determined by the necessity of providing adequate interior space. As a result, tall buildings and high-building densities, block views of landmarks. Zoning regulates land use by protecting the interests of individuals who invest in an urban community. Therefore, to preserve the views of these landmarks, an appropriate zoning ordinance on building height and density must be established.

During the last two decades, a considerable number of studies have aimed to refine viewshed analysis. Viewshed analysis identifies all areas on a terrain's surface that are visible from a given observation point by specifying the objects which block the views. Felleman (1986) shows how viewshed studies can play a major role in scenic analyses. Blair (1986) analyzed the area from which a high-rise building is visible to evaluate its effect on the visual environment.

Recent progress in computer graphic techniques allow viewshed analysis to be carried out more easily. Geographical information systems (GIS) and computer-aided design (CAD) tools, including three-dimensional raytracing approaches, are some of the means that are currently used to identify a viewshed in a landscape (see, e.g., Glassner 1989; Fisher 1991, 1992, 1993). Such tools are able to deal with actual building layout, vegetation and elevation for the entire area under consideration. Since the advent of GIS, many articles have been devoted to the study of visibility analysis, as discussed in Jones (1997) and Hanna (1999). Sorensen and Lanter (1993) proposed two procedures to reduce data-structure-induced error in grid-based viewshed analysis. De Floriani and Magillo (1994) and Nagy (1994) analyzed viewshed maps using triangulated networks rather than rectangular grids for general applicability. Lee (1994) considered the visibility dominance, which occurs when all visible regions from an observation point are also visible from another observation point. Aguilo and Iglesias (1995) studied the visual ellipse. This shape correlates with the viewshed, preserving its main orientations and area. Lee and Stucky (1998) examined least-cost paths on grid-based data using viewshed tools. Landscape Institute and Institute of Environmental Management & Assessment (2002) described how landfills are visible not only to those who live nearby but also those who pass by on nearby road. A brief overview of viewshed analysis using GIS tools can be found in O'Sullivan and Turner (2001).

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