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Accessible Plots: A configurational approach to urban diversity

Paper presented at the International Seminar of Urban Form, Stockholm, 2006.

Lars Marcus



Keywords: diversity, plot-structure, urban space, urban morphology, description, axial map.

 

1. Introduction

More than forty years ago Jane Jacobs wrote the most influential book on urban development in the post war era.[1] Immensely clear-sighted and poignant in its criticism it was not, although often intuitively correct, as strong in theoretical depth in its suggestions for remedies. It can be said that one of her four main suggestions, the idea of short blocks and its importance for through movement and inhabited streets, has within space syntax research been lifted from the intuitive level of Jacobs to an analytical theory under the name of  “The theory of natural movement” (Hillier et al, 1993).

         Another main suggestion of Jacobs was the idea of buildings of varying age and their importance for the diversity of uses within cities. In this paper a series of earlier papers are summarised and further developed, trying to also lift this suggestion to the level of an analytical theory, possibly called “The theory of natural diversity”. Marcus et. al. (1999) presented the theoretical foundation for such a theory where the importance to focus the spatial level of plot-structures in cities was argued. The plot-structure is in a direct way a representation of the distribution of landowners and thereby fundamental actors in the city, which act according to different strategies. Many actors can then lead to many strategies that in effect can lead to a higher diversity, it was argued. Marcus (2001) presented empirical support for such a theory, showing convincing correlation between the density of plots and the number of people renting space for economical activity and in extension the number of present lines of businesses.

         In these empirical tests plot-density was measured as a density of plots either for an urban block or street. Thereby it was fundamentally a geographic description, aggregating values on a geographic unit, that is, there were no analyses of the distribution of space itself, the latter being the fundamental aim within space syntax.[2] A new GIS-based application, called the Place Syntax Tool, has opened the door to a morphological approach to the same issue, where the axial map developed within space syntax is instrumental (Ståhle et al, 2005). In effect what is measured is ‘accessible densities’, that is, densities that through the axial map take into account densities of other geographical units in the system.

         In this paper it is used to measure and correlate plot- and diversity-densities, where the results supports the general theory suggested above not only for economical parameters but also social, such as age groups.

 

2. The plot as an example of legally defined space

In Ståhle et. al. (2005) it is argued that one of the reasons for the success of space syntax is that it has seriously focused the geometric description of urban space. Not only by stressing the importance of a configurational view on urban space but also by developing new geometric units by which such configurational representations can be constructed, most important of these being the axial line.  What is different with the axial line from most geometric units within spatial analysis is its strong relation to the experience of individual human beings in space. By capturing the potential accessibility and visibility for a human being in a particular space the paper suggest that it can be seen as a contribution to a general field possibly called anthropocentric geometry.

         Another way of putting this is to say that any form of human geography, including urban morphology, needs a conscious development of a ‘human geometry’. That is to define and develop the subset of geometry relevant for its discipline, apart from the mathematically possible. This is of course exactly what takes place within urban geography but contributions like the axial line possibly point to the possibility of developing this into a theory of its own, where different geometric items and models sort under different categories and have different uses.

         Another category of what could be called ‘human geometry’ then are what within urban geography is referred to as ‘domains’ usually represented as polygons within urban areas. These are usually defined as areas that in one way or another are under the legal control by some kind of an urban actor. It can be a private person, a developer of even the municipality in itself. It can also have to do with control by ownership like in the case of a private property or some legal control of what can happen within a domain, for example planning regulations stipulated by the municipality. All these types of domains can be sorted under a different category of human geometry, which in this case is legally defined rather than experientially defined.

  

Figure 1. Examples of human geometry: experientially defined space (left) and legally defined space (right).

 

It is exactly this category of human geometry, more specifically the privately and publicly owned domains we call plots or properties, we suggest are necessary to focus to investigate the possibility for such a thing as natural diversity (Fig. 1).

         The reason we focus the particular domain of the plot or the property is that it represents the presence of an actor in urban space and furthermore the location of the influence of that actor in urban space. Within this domain the actor is free to act, obviously keeping within the frames set by laws or rules of a higher order, for example the planning regulations set by the municipality or the laws set by the state. The influence of the latter two have, most interestingly, strong geographical limits.

         Such actors normally will develop particular strategies for their actions. An area with comparatively many plots then seem to have the potential to carry more such actors and thereby more strategies for acting and furthermore more diverse such strategies. In the end such an area seem to be carrying the potential to more easily develop a diverse content than an area with comparatively few plots and hence few actors and strategies. It is this hypothesis that makes us believe that we here have a link between urban space and urban diversity. Obviously other things like land-use regulations can override the effect of space here but what we are trying to determine is the particular influence of space.

 

3. Empirical investigation A: five work areas in Stockholm

To empirically investigate the degree to which the distribution of plots determine the distribution of actors in the above sense, Marcus (2001) presented a study of five work areas in Stockholm. What was correlated was the density of plots per block, first with the density of actors renting floor space in the same block, then with the density of lines of businesses in the same block. The degree of correlation varied between the different work areas but to summarize they are here taken as one population, constituting 69 items, that is 69 blocks.


Figure 2. Correlation between density of plots and density of floor space renters (left) and correlation between density of plots and density of lines of businesses (right).


First (Fig. 2), we have the correlation between spatial capacity and secondary actors, that is businesses renting land or floor-space from primary actors. The correlation turned out to be quite strong (R2= 0.61) when leaving out three items that diverted strongly. The correlation including these three items was R2=0.47. We found this to be a surprisingly strong correlation that seemed to support the idea that plot density had a significant impact on urban diversity.

         Secondly we correlated the plot density with the amount of lines of businesses represented in each and every block. This also resulted in a strong correlation when leaving out the same three items as earlier (R2=0.66). (The correlation including these three items was R2=0.54.

         Thus it seemed that there is a significant influence of land division itself, here expressed as plot density, on urban diversity. This seems to confirm the possibility of such a thing as ‘natural diversity’, that is the proportion of diversity that is determined by space itself, rather than by the presence of specific briefs or regulations.

 

4. Developing a more robust measure

A weakness of spatial capacity as measured here is its strong local character. We could see how the odd block could significantly disturb the overall correlation pattern. It therefore seems interesting to look for possibilities to combine this measure with more global measures. One obvious such measure is spatial integration, which in several studies have been shown to have significant impact on not only movement but also the distribution of land-use in general. Since the density of plots seems to influence the distribution of diversity, this could possibly be a way of developing a more robust tool to capture this, and possible also making it applicable in urban planning and design.

         In Ståhle et. al. (2005), the development of a new GIS-application called the ‘Place Syntax Tool’ is presented. It makes it possible to use the axial map to measure accessibility within urban space. It can either be used by itself or combined with more conventional measures such as metric distance. In Ståhle (2005) quiet sensational findings concerning the accessibility to green areas in Stockholm are presented, where the measurement of accessibility using the Place Syntax Tool proves to correlate with people’s actual experience better than conventional measurements. In the present paper the same measurement is tested to see if it can be developed into a more robust measurement of urban diversity, building on the earlier studies.

 

Figure 3. Geographical representation of the distribution of population (left) and configurational distribution of population (r=10) (right).

 

5. Empirical investigation B: The city-district of Södermalm in Stockholm

The city-district of Södermalm was chosen as area of study since it represents a generally dense inner-city area though morphologically quite varied. It furthermore has earlier been studied in depth using integration analysis (Marcus 2000) so that links between the two types of studies easily can be conducted.


Figure 4. The distribution of spatial capacity, measured as accessible density of plots (top), correlating with accessible density of lines of businesses by 40% (bottom left) and accessible density of age groups by 69 %  (bottom right).

 

When this technique was tested, what in effect was measured was the accessibi­lity to plots from each and every plot within three axial lines, divided by the amount of accessible plot area within the same radius. These measures were then correlated to the accessibility to both economic and social indices of diversity, where the economical index in this case was lines of businesses, and the social index was age groups. It turned out that spatial capacity correlated to 40% with the economical index (R2=0.40), and to 69 % with the social index (R2=0.69),[3] implying that the higher spatial capacity within a radius from a plot, the more lines of businesses and age groups within the same radius, that is, the higher diversity.

 

6. Conclusion and discussion

This means that we could reproduce the convincing correlations from the earlier study. It furthermore was conducted in a mixed area where some plots were used solely for economical uses and others solely for residential uses. The study area and thereby the population was also much larger than in the earlier study, consisting of some 1700 plots and no items were left out. From this one could conclude that we truly had found a more robust measurement of urban space that captured urban diversity, that is, exactly what we were looking for.

 

Fig 4. Spatial integration of Södermalm (left) and spatial capacity of Södermalm (right).

 

We here then see the grounds for a theory of natural diversity that can accompany the theory of natural movement. The one showing how urban form has an impact on pedestrian movement and the other showing how urban form has an impact on diversity in the ‘content’ of urban space.[4]

 

References

Hillier, B., Penn, A., Hanson, J., Grajewski, T. & Xu, J. 1993. Natural movement: or, configuration and attraction in urban pedestrian movement. Environment and Planning B: Planning and Design, volume 20, s. 29-66

Hillier, B., Space is the machine, Cambridge University Press, 1996.

Jacobs, J. 1961, The death and life of great American cities, New York.

Marcus, L, Steen, J. 1999, ’Physical planning for economic growth — a study of urban areas’, Proceedings Second International Space Syntax Symposium, Universade de Brasilia, Brasilia.

Marcus, L. 2001, ‘The impact of land-division on long-term occupation – the possibility of such a thing as natural occupation’, Proceedings Third International Space Syntax Symposium, Georgia Tech, Atlanta

Marcus, L. 2000. Architectural Knowledge and Urban Form – The functional performance of Architectural Urbanity. TRITA-ARK Akademisk avhandling 2000:2. KTH.

Marcus, L. 2001, “The impact of land-division on long-term occupation – the possibility of such a thing as natural occupation”, Proceedings Third International Space Syntax Symposium, Georgia Tech, Atlanta.

Marcus, L. 2006a, “Urban form and sustainable cities”, Journal of Urban Design, submitted.

Marcus, L. 2006b, “Spatial capital – an outline of an analytical theory of urban form”, in New Urbanism and Beyond, Rizzoli, forthcoming.

Ståhle, A., Marcus, L. & Karlström, A. (2006) “Place Syntax – Accessibility with axial lines”. Environment and Planning B: Planning and Design, submitted.

Miller, H. J. 2000, ”Geographic representation in spatial analysis”, Geographical Systems, 2:5



[1] Jacobs 1961.

[2] The concepts of distribution of space, distribution in space and distribution through space, are developed in Koch, 2005.

[3] It is important to stress that the population that is correlated here consist of no less than 1700 plots, encompassing a complete inner city district including some pure residential areas. Against that background the correlation for the economical index is surprisingly high rather than low. Furthermore, by excluding 17 out of these 1700 items, the correlation rises to R2=0,60, which tell us that the correlation is fundamentally strong. Further and more detailed investigations on these promising correlations are currently under hand.

[4] These two are combined in amore general theory called ‘The Theory of Spatial Capital’, Marcus, 2006b.

© LARS MARCUS
architect and professor in Urban Design

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