The Dictionary of Human Geography (22 page)

census tract
A small areal unit, containing a few thousand residents, used to collect and report census data. The first tracts were defined by the US Bureau of the Census in 1920 to approximate natural areas or neigh Bourhoods, providing useful data for anal ysing urban social geography (cf. sociaL area anaLysis). Many censuses now use a similar spatial architecture with varying termi nology; some report data for areas with only a few hundred residents. Most tracts and com parable areas are designed for logistical con venience, although those for the 2001 England and Wales census were defined by geographers to produce areas with different housing characteristics (Martin, 2002). rj (NEW PARAGRAPH) Suggested reading (NEW PARAGRAPH) US Bureau of the Census, Census Bureau Geog raphy. http://www.census.gov/geo/www/GARM/ (NEW PARAGRAPH)
central business district (CBD)
The nucleus of an urban area around its most accessible point, containing an internally differentiated concentration of retail and office establish ments. In cities where most workers and shop pers travel by public transport, the CBD has the highest density land uses, most valuable land and is the focus for most intra urban jour neys. With greater reliance on private transport, deceNTEaLizatiON and deconcentration trends are eroding the CBD's role: most are now (NEW PARAGRAPH) declining both absolutely and relatively as shops and offices move to more accessible suburban locations (cf. edge city; retailing; sprawL). rj (NEW PARAGRAPH) Suggested reading (NEW PARAGRAPH) Carter (1995); Murphy (1972). (NEW PARAGRAPH)
central place theory
A theoretical statement of the size and distribution of settlements within an urban system in which marketing especially retaiLing of goods and services is the predominant urban function. The theory assumes that both customers and retailers are utiLity maximizers, making it a normative statement against which actual patterns can be compared. (NEW PARAGRAPH) Of the two separate central place theories developed, Christaller?s (1933) has been most influential. It was based on two concepts: the range of a good the maximum distance that people will travel to buy it; and the threshold for a good the minimum volume of sales neces sary for a viable establishment selling that good (or a bundle of linked goods, such as groceries). In order to maximize their utilities, retailers locate establishments to be as near their customers as possible and customers visit the nearest available centre: in this way, expend iture on transport costs is minimized and spending on goods and services maximized. (NEW PARAGRAPH) On a uniform plane with a uniformly dis tributed population, Christaller showed that application of these two concepts produced a hexagonal network of central places housing the establishments, organized in a hierarchy whose number of levels reflected the number of goods/services with similar range and threshold values (he identified seven). Each was centrally located within its hinterLand, with those at the hierarchy?s lowest level hav ing the smallest number of establishments and serving the smallest hinterlands, and thus most widely distributed. The ways in which smaller settlements nested within the hinter lands of larger ones depended on a further set of principles. Christaller identified three (as shown in the figure). The marketing principle (k ? 3: a in the figure) minimizes the number of settlements so that each is at the meeting point of three hexagonal hinterlands for centres at the next hierarchical level up: the number of centres in each order is 1,2, 6, 18, 54, 162 and 486. According to the transport principle (k ? 4: b in the figure) the goal is to minimize the length of roads joining adjacent places. Each settlement is located centrally on the boundary line between the hexagonal hinterlands of two places in the next highest order hierarchy, and the number of centres is in the ratio 1, 2, 8, 32, 128, 512 and 2,048. Finally, the administrative principle (k ? 7: c in the diagram) has each lower order settlement and its hinterland nested exclusively within the hinterland of a single settlement in the next highest order, producing a much larger num ber of places in the ratio 1, 6, 42, 294, 2,058, 14,406 and 100,842. (NEW PARAGRAPH) Losch?s (1940) model was much less restrict ive than Christaller?s. Rather than bunch all functions into seven ?orders? he treated each as having a separate range, threshold and hex agonal hinterland. Where feasible, functions were clustered into settlements but all central places with certain functions in them need not as in Christaller?s scheme also contain all ofthe functions with smaller ranges and thresholds. This produced a much wider range of settle ments in terms of size and complexity of busi ness profiles: whereas Christaller?s theory produced a stepped hierarchical urban system, Losch?s was consistent with a more continuous distribution of urban sizes. (NEW PARAGRAPH) Central place theory was a major stimulus to work in the early years of geography?s quanti tative revoLution: it was described by Bunge (1968, p. 133) as ?geography?s finest intellec tual product?. Christaller?s work, in particular, was the basis of much research into the size and spacing of settlements and into consumer behaviour (both inter and intra urban), and also as the basis for planning settlement pat terns not only in the anodyne cases of new settlements in the Dutch polders and the dis tribution of new shopping centres in cities, but also in the violent resettlement of Eastern Europe as part of the Nazi hoLocaust. (NEW PARAGRAPH) With greater mobility and customer choice available in many, more developed, societies, the underlying assumptions are increasingly irrelevant and the theory remains more as an exemplar of modelling during that period of geography?s history than as a paradigm for understanding contemporary settlement pat terns, although it is one of the theories ?redis covered? in the new economic geography. (See also periodic market systems.) rj (NEW PARAGRAPH) Suggested reading (NEW PARAGRAPH) Beavon (1977); Berry and Parr (1988); Fujita, Krugman and Venables (1999). (NEW PARAGRAPH)
centrifugal and centripetal forces
Terms adapted from physics by C.C. Colby (1932) to describe two counteracting forces generat ing intra urban land use changes. Centrifug central place theory The size and spacing of central places, plus their hinterlands (left) and routes (right), according to three variants of Christaller's model: (a) the market principle, which minimizes the numbers of centres: (b) the transport principle, which minimizes the road length: and (c) the administrative principle, in which (NEW PARAGRAPH) hinterlands are nested hierarchically forces push residential, business and other users away from the congested, polluted, high density and expensive inner city areas towards the suBurBs and beyond (cf. decon centration: see counter urBanization; (NEW PARAGRAPH) decentraLization; sprawL), whereas centri petal forces attract them towards the centre for the benefits of accessiBiLity and aggLom eration (cf. gentrification). The balance of these two forces at any time determines the changing urban morphoLogy. rj (NEW PARAGRAPH)
chain migration
A term used to describe migration that occurs in a sequence, when the movement of one person causes others to follow. It is a major component of network based theories of migration. Chain migration typically begins at the family scale, with a sin gle person moving to a new place in search of better opportunities. Once settled, that person facilitates the migration of other members of the nuclear or extended family (Hugo, 1994). In time, the migration network extends to friends and acquaintances as well. As this occurs, migration becomes self perpetuating. New arrivals are assisted by those who have already learned to cope in the receiving society (Castles and Miller, 2003). This is particularly effective in the process of finding shelter and work, and newcomers generally live in close proximity, and take similar jobs to those who help them. This process leads to the develop ment of immigrant encLaves in housing and labour markets, as Banerjee (1983) has shown in the case of internal migration in India (cf. segregation and segmented LaBour market). Chain migration also fosters financial trans fers, or remittances, between immigrants and family members who are still residents of the source country, as well as other forms of trans nationALism (Levitt and Glick Schiller, 2004). (NEW PARAGRAPH) From the point of view of receiving coun tries, the chain migration process can be thought of as an ?immigration multiplier', in the sense that each individual immigrant is likely to generate the entry of several others over time (Jasso and Rosenzweig, 1986). Once established, these networks develop routinized systems of movement that can even circum vent state authority (Bocker, 1994). It is widely believed, for example, that chain migration has been an essential ingredient in the entry of the approximately 12 million undocumented immigrants residing in the USA. This insight has often been used by critics of immigration, who argue that the combination of the self perpetuating nature of chain migration and the multiplier effect will cause a geometric increase in immigration (cf. Goering, 1989). However, it is worth remembering that social networks are just one contributing factor in migration, and that other factors, notably economic differentials between countries and various forms of vio Lence, are structural causes of migration. dh (NEW PARAGRAPH) Suggested reading (NEW PARAGRAPH) Castles and Miller (2003); Hugo (1994). (NEW PARAGRAPH)
chaos theory
A branch of non linear math ematical theory dealing with dynamic systems which exhibit aperiodic behaviour that is sen sitive to initial conditions and is unpredictable in detail. Such sensitivity to initial conditions is often referred to as the ?butterfly effect?, with the illustration that the flapping of a butterfly?s wings in one part of the world might, through tiny impacts on the atmosphere, have major impacts elsewhere in the world. The behaviour of systems that exhibit chaos may appear to be random, even though the system is deter ministic in the sense that it is well defined and contains no random parameters. This tech nical use of the word ?chaos' is at odds with everyday language, which suggests complete disorder. Chaotic systems are ?orderly? in being deterministic, and also usually have well defined system structure and statistics. A very simple example of such a system is provided by May (1973) in his standard logis tic model of population growth: (NEW PARAGRAPH) XI+1 = aXt [1 bXt/a], (NEW PARAGRAPH) where next year?s population Xt+1 is depen dent on the current population Xt and par ameters a and b. The system exhibits very different behaviours depending on the values of a: if 1
4, then there is a divergence to minus infinity (a total collapse of the system), but for values of a between 3 and 4, there are interesting dynam ics: if 3
Chicago School
The first sociology depart ment in the USA (founded in 1892), located at the University of Chicago, where scholars including Robert Park, Louis Wirth and Ernest Burgess established an agenda, approach (human ecology) and methodology for the study of urban areas. From the 1910s through to the 1930s, the scholars at the Chicago School set out to study the city as ?a product . . . of human nature' (Park, 1967 [1925], p. 1). Indeed, the Chicago School sociologists saw the city as ?the natural habitat of civilized man [sic]? (Park, 1967 [1925], p. 2). Fundamental to their theories about urban life was an expectation of social organ ization and control. They anticipated that land use patterns in a city would reflect ?an orderly and typical grouping of its population and institutions' (Park, 1967 [1925], p. 1), and sought to study these systematically. Chicago School sociologists developed detailed descriptions of urban life based on field observations of Chicago. In doing so, they advocated that ethnographic methods drawn from anthropology be applied to urban cultures (Park, 1967 [1925]). Their in depth observations were hindered by a ten dency to generalize from the single Chicago case, but their emphasis on observation remains influential in community research. (NEW PARAGRAPH) The Chicago sociologists drew upon Darwin's theories of order and ?cooperative competition? among species in a shared terri tory and applied them to humans in urban environments (Park, 1936). The concept of community articulated the biotic level of social organization, which correlated for the Chicago School scholars with race and ethnicity (Theodorson, 1961; Knox, 1994). Social communities as defined by ethnicity formed ?natural areas' that were segregated from one another (Park, 1967 [1925]). By applying bio logical metaphors to sociology, Park and his colleagues were creating a scientific justifica tion and legitimation for the study of social phenomena (Entrikin, 1980). At the same time, however, their naturalizing of racialized social communities fostered and reinforced notions of a ghetto that was both voluntary and temporary: ?they never saw the difference between the ethnic enclave and the black ghetto? (Philpott, 1991 [1978], p. 141). Further, Philpott argues that with the excep tion of the ?Black belt' African American ghetto, the natural areas (social communities) about which the Chicago sociologists wrote were never as homogeneous as some of their writings suggested. (NEW PARAGRAPH) Well known and influential studies of ?nat ural areas' that were developed by the Chicago (NEW PARAGRAPH) School faculty and their students include Zorbaugh?s The gold coast and the slum (1929) and Frazier's ?Negro Harlem: an ecological study? (1937). Perhaps the most famous among the writings of the Chicago School, however, is that of Ernest Burgess on ?The growth of the city? (1967 [1925]). In it, he offered a descriptive model of urban structure to explain land use, urban growth and neigh bourhood change. He posited urban expan sion based on differentiation of land uses and competition among those uses.(a basic premise also articulated in the von thunen model and also other land use models by Hoyt (1939) and Alonso (1960), among others). His zonal model represented the city as a series of ?con centric circles', or zones. The central zone at the core of the city was the central business district (cbd), called the ?loop? in his model due to the influence ofChicago as the empirical case study. Successive zones were described as residential areas, differentiated from one another based on categories of ethnicity, social class and housing type. (NEW PARAGRAPH) The process of urban expansion was explained by Burgess (1967 [1925]) in terms of the invasion and succession of one zone (predominant land use) into the next outer zone adjacent to it, with physical expansion of the city the result. The mobility assumed in the model to be inherent to urban expan sion was seen by Burgess to be both a stimulus to urban growth and the source of instability, especially in lower income and immigrant communities. Thus, crime, poverty, home lessness and social and psychological instabil ity were seen as naturally occurring phenomena in the zone of transition just outside of the expanding CBD (Burgess, 1967 [1925], p. 54). (NEW PARAGRAPH) Burgess' model, and the overall goals of the Chicago School to study urban life, com munity and organization, have been extremely influential in urban studies generally, includ ing urban geography. The inherent (and explicit) spatiality of Burgess? model of urban growth is appealing to geographers. Terms such as CBD are ubiquitous in the field. Yet the assumptions underlying the model and its theory of growth, particularly the naturalization of race, ethnicity and social problems such as crime and homeless ness, limit its use and applicability. (NEW PARAGRAPH) The Chicago School is clearly situated in and thus limited by its time and place because of its reliance on human ecology and Darwinian metaphors, and upon the city of Chicago as the main case study. Yet Park?s (1967 [1925]) original description of an (NEW PARAGRAPH) agenda for research included popuLation and demography, Land use, patterns of home ownership and migration, community devel opment and character, neighbourhood history, occupational and class mobility, social unrest and social control (including poncing and urban policies). Many of these topics remain of vital interest to urban geographers, and draw upon ideas from the Chicago School about pattern, process and community, although our contemporary approaches to and theories of these topics are necessarily different. dgm (NEW PARAGRAPH) Suggested reading (NEW PARAGRAPH) Dear (2002); Entrikin (1980); Jackson and Smith (NEW PARAGRAPH) ; Park, Burgess and McKenzie (1925). (NEW PARAGRAPH)