Ref 006 Visibility and Spatial Use in Urban Plazas
A Case Study from Biskra, Algeria
Yassine Bada
阿拓莫兰University Mohamed Khider-biskra, Department of Architecture, Biskra, Algeria
yassinebada@yahoo.fr
Yasemin Ince Guney
Balikesir University, Department of Architecture, Balikesir, Turkey
yince@umich.edu
Keywords
大脑肥胖症spatial configuration; urban analysis; urban territoriality; spatial use; urban plaza; Biskra
Abstract
This paper springs from the assumption that a good space is a used space and people's behavior, movement and use is directly related to the visual fields created by the spatial visibility. It is also assumed that the use of space is governed by two major factors: Its attractiveness in terms of movement, i.e. how easy for people to move in as its being to-movement and through-movement space; and the conduciveness of its visual properties for people's activities. This paper aims to examine the correlation between the spatial use and isovist properties of plaza spaces with a case study from Algeria. The paper first starts with a comparative study of four urban plazas within the city center district of Biskra, Algeria, in which the correlation between the number of static people in sitting and standing positions and the syntactic and isovist properties of each plaza is examined. The second part of the paper focuses on one of these plazas and explores why within it some parts are busier and more preferred by people then others. Believing that a space may have many sub-settings and that its use is determined by the affordances of these sub-settings, the plaza has been subdivided into subspaces based on its physical properties. Each subspace has been investigated further to look for links between visibility and spatial use. The Depthmap program, developed by Alasdair Turner, has been used to observe the visibility properties while behavior mapping, i.e. people counting and spatial use mapping, has been used to observe the spatial use. The results indicated that spatial use is strongly related to visual considerations relevant to the type of activity and that people might undergo some discomfort for visibility matters.
Introduction
Urban space is the "void" between buildings such as streets, plazas and parks that enable movement and outdoor activities of the general public. It is considered as the "scene" of the city life and has been the center of different theories of urbanism and city planning through history. Nowadays, the concepts of sustainability driven by environmental quality, public health and people's well-being put more emphasis on the quality of public life as exemplified by the set of recommendations and laws put forward to improve public spaces.
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The quality and the success of urban spaces such as public plazas is generally investigated either by looking for how comfortable are these spaces in term of ambience characteristics such as thermal and acoustic comfort, i.e. more sensory oriented approach, or by questioning and exam-ining the spatial properties that are considered as qualitative such as enclosure (Sitte, 1945), good proportion (Alexander, 1977) and the landscape features like fountains and benches. However, questioning and examining the use of and people's behavior within these spaces could be infor-mative on what matters most for people and what guides their activity patterns within these spaces. An earlier example is renowned William Whyte's study of New York urban plazas to
understand why some were underutilized while others were crowded. Accordingly, Whyte con-cluded that plaza use is correlated mostly with the amount of sittable space and its proximity to street life and suggested specific guidelines for the design of successful plazas (Whyte, 1957). Alexander (1977), on the other hand, based on his investigation of the pattern of occupancy of public spaces, argued that people avoid spaces that are either too exposed or too enclosed and that space occupancy ensues around the borders and edges that people prefer. Defined by Gehl (1987) as the "edge effect," once the edges are full, the occupation goes gradually inwards.
As mentioned by Campos (2005), earlier studies failed to link people's behavior and space use to spatial properties of the spaces such as their morphology and visibility properties. In space syntax approach, it is suggested that better locations for unprogrammed static use do not depend on the provision of specific attractions or facilities, but may be associated to "the visual properties of space experienced by the stationary person" (Hillier et al, 1990). Experiencing a space is indeed experiencing the visual fields generated by the arrangement of buildings and urban components. Since experiencing space involves motion, space is more dynamic than static; it is a succession of subspaces involving changing visual fields (Thiel, 1970). This experience definitely impacts people's aesthetic appraisal of as well as behavior in urban spaces.
江苏中行Visibility and visual perception are of a great importance on how we behave, appreciate and experi-ence the environment and could be determinant factors in designing urban spaces. To represent space as experienced by the perceiver and to quantify the spatial environment through a set of measurements the concept of isovist is introduced by Benedikt (1979). The drawback in Benedikt's method was that it only considered the local properties of space omitting the visual relationship between current location and the whole spatial environment (Turner, 2001). Turner suggested that since the way an individual experiences and uses space involves motion, the experience of space is related to an interplay of isovists and that more than one isovist is needed to quantify the actual perceived space experienced by the individual (Turner, 2001). Turner also developed a software package called Depthmap capable of performing Visibility Graph Analysis (VGA).
Campos (1999, 2005) has used overlapping point isovists as a tool for understanding preferable location of static people in public spaces in tweleve squares the city of London and found that the static occupancy of a public space is a function of their spatial configuration and their local inter-connectivity in the urban fabric where they are embedded. Papargyropoulo (2006), on the other hand, in a comparative study of Regent's Park in London and Pedion Areos Park in Athens, found that the most integrated spaces are more used by organized activities performed by large groups, w
信息学hile other stationary activities such eating, reading and relaxing take place more on visually secluded spaces. Although the physical qualities of spaces such as comfortable sitting features or existing of shade are important factors in people's use of plazas, the current study focuses on the importance of visi-bility and investigates the relationship between visibility and spatial use in the selected plazas in Biskra, Algeria. The investigation considered stationary positions of sitting or standing without con-sidering the specific type of activities being performed in these positions. Moreover, based on the assumption that environments are composed of behavior settings, we believe a plaza could be subdivided into a number of subsettings and that the liveliness and the use of the plaza are depen-dent on the potentialities afforded by those subsettings. Accordingly, the present work subdivided the studied plazas into several subspaces. The next section will introduce the case studies.
Case Study
Biskra is an oasis city located in the southeastern part of Algeria. It is known by its hot and arid weather. The urban structure of the city is a compilation of principally three different urban fabrics: the colonial fabric with its gridiron characteristics, the irregular and dense fabric as an extension of the old nucleus of the city and the 'loose' and amorphous modern fabric of the post-independence urban development. The main open spaces such as plazas and public gardens encompassing the p
ublic life are situated within the city center, which is part of the gridiron fabric and expanding to the edge of the irregular and dense fabric. Most of the plazas were initially created as "left over" spaces or as a result of coming together the two geometrically different urban fabrics, which were
then redesigned to be public spaces (El Houria , Dalaa and Zwaka plazas as case study). It should also be noted that the plazas of the gridiron fabric such as one of the cases in this paper, Larbi Ben Mhidi plaza, were designed as public spaces in the first urban planning scheme of the city during the colonial period.
The plazas to be investigated are El Houria, Zwaka, Dalaa and Larbi Ben Mhidi Plazas (Figure 1). The selection of plazas was guided by the following considerations: their location, size, and the different ways in which they are linked to their surroundings.
Figure 1
Plan of the city center district, showing the four plazas and important streets nearby: La République street, (2) Dalaa Plaza, (3) Larbi ben Mhidi plaza, (4) Emir Abdelkhader Boulevard, (5) El Houria Plaza, (6) Hakim Saadan street, (7) Zwaka Plaza.
The first plaza, El Houria, was selected for its location which enables it to be used frequently and for
the services available such as cafes and shops. On two opposite sides of the plaza, two main busy streets go parallel to each other. The plaza layout is mainly characterized by its two-level space: the higher section limited by El Emir Abdelkader Boulevard, the most livable street of the city and the lower section limited by a busy street that links to a popular market. The height dif-ference between the two levels is about 1.20 meters, and the connection is assured by a series of stairs without causing any visual obstruction within the plaza. The space layout is characterized by a linear fountain along a central pathway, a memorial wall and well defined spaces by built-in sitting areas. Surrounding land uses include cafes, few shops and a state building. In the present study, all the features contained within this plaza do not obstruct eye-level views for pedestrians such as sitting areas, fountains and trees are not considered. El Houria plaza is the one to undergo further investigation of correlation between the visual properties and the spatial use. Dalaa Plaza is typically characterized by its irregular shape resulting from the presence of a rocky hill within the built area. The plaza is linked to its surroundings by a number of streets. Its layout includes some fountains, sitting places and mixed-use of boutiques and cafes.
Zwaka Plaza is situated within mostly a residential area, Hakim Saadane, one of the major streets of the city, passes through the plaza and constitutes the major link to the rest of the city. The layout of t
he plaza basically involves a green space crossed by a pattern of pathways together with a fountain, some sitting places, and little service activities.
Larbi Ben Mhidi Plaza is situated within the gridiron fabric near a covered market. It is linked to its surroundings by eight streets and its layout is mainly covered with a bunch of trees in its core that give it a garden like plaza. The mixed–use activities of the plaza consist of boutiques, cafes, and a bank, make it a livable place.
In Situ Observation
The observation conducted in this study is stationary people counting. To make the observation and people counting easier and more accurate plazas have been subdivided into zones with detailed layout plans. For each plaza and for each sub-zone in each plaza a different observer is assigned. Observations were done on two days; a weekday and a weekend, in sunny days on August 2008 in the late afternoons from 7pm to 8pm. Since weather is believed to affect static behavior, although does little on natural movement (Hillier et al, 1992), the observation times were chosen when the weather was conducive to outdoor activities in order to avoid the impact of the climatic factors. All the plazas were observed at the same time. During the observation of stationary activities, static peo
ple were sorted into two categories: those sitting in a formal sitting place and those in standing position or sitting in informal places such as on tree borders or flower-bed walls. The type of activity people are engaged in during observation time is not considered as it is irrelevant to the present study. People counting has been done according to people position within space (detailed plan are given to the observers, and plaza pavement are taken as landmark to help to locate people accordingly). During the hour observation, first, static people are counted, then people coming (stopping) are added to the count , however no interest for people leaving the space. The goal of the present study is to see which space is most visited . But it is important to mention that the number of people counted during the whole hour observation (first counting added to people coming) is largely equivalent to the number of people counted at the beginning of the observation.
Table1
The number of static people and occupancy rate for both categories sitting and standing position according to time period for the four plazas.
In this study, the total number of static people is considered for all the plazas, while El Houria plaza is selected for further examination. The first part of the paper aims to compare the occupancy of the f铸造论坛 our plazas considering the number of users, i.e. all the static people without an interest of their position, and to see their correlation with isovists measurements. Thus, first assessment of the data collected about the amount of use of each plaza considering only the number of static people during the period from 7 pm to 8 pm (after working hour), shows that EL Houria and Dalaa plazas are the most used and preferred for this period of time (table 1). Meanwhile both Larbi Ben Mhidi and Zwaka plazas are underused. In comparing the occupancy rate Here it should be mentioned that these results are neither plaza name no. of people Plaza area (m2) Occupancy rate; people/m2 El Houria 514 11205.969 0.0458
Larbi Ben Mhidi 108 2513.056 0.0429 Zwaka 763 10757.862 0.0070 Dalaa 308 8718.236 0.0353
conclusive nor exhaustive and object of on going work. However they could give some clues about the plaza uses, albeit limited for a certain period of the time, and help promote questions regarding the relationship between visibility and spatial use of urban plazas.
Syntactic Analysis of the Four Plazas
It is believed that there is a correlation between spatial configuration, i.e. intelligibility of the space, and people's behavior, movement and use of these spaces. According to Hillier (2005), spatial config
uration of the street network shapes movement of people and that the position of a street in the overall grid affects to-and through- movement on this street. As the network of streets provides the means for people to get to the plazas, our goal is to examine the accessibility and attractive-ness of the location of each plaza within the broader network, i.e. city center district, using space syntax techniques.
Table2
Syntactic and isovist measurements of the plazas within the perimeter of the city center district and within the perimeter of 250 meter radius of their center, and occupancy rate.
Two sets of measures are considered. The first measures are those relevant to spatial properties of each plaza: global measures of integration and relativized entropy that indicate how ordered the a system is from a location, and local measures of connectivity and visual control that indicate the degree of choice each space represents for its neighbors to move into. The second measure is the visual access that is the degree to which different places and features can be seen and also the location from which people in a particular environment can see particular places and features (Montello, 2007). The visual access measure is examined using point isovists taken from the most integrated streets that surround or junctions that lead to the plazas to see to what degree is their overall layout is visible. The syntactic measures are calculated, first, considering the plan of the city center district including all four plazas examined, and then considering each plaza by itself within its surrounding of 250 meter radius from its center.
Syntactic analysis has been carried out for the plazas examined using Depthmap 07 software program developed by Alasdair Turner from UCL. The results of the analysis of the plan of the city center district including all four plazas and those of each plaza within its surrounding area have been given in Table 2. A comparative interpretation of the different measures reveal that, in terms of integration value, El Houria plaza is the best integrated within the system with a value of 0.78 and th
e least integrated is Dalaa plaza. However, in the local measure, connectivity both El houria and Zwaka plaza have the higher connectivity value.
The visual integration (HH) map provided in Figure 2 , highlights the high value of two streets (dark color), El Emir Abdelkader Boulevard passing by El Houria plaza and closer to Dalaa plaza and Hakim Saadan Street passing by Zwaka plaza and closer to Larbi Ben Mhidi plaza, where La Repuplique Street connects Hakim Saadan Street to this plaza. As can be observed from the figure, El Houria Plaza is remarkably integrated within the system. The map also indicates that
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