Source: adapted from H. Carter (1995) The Study of Urban Geography, Fourth Edition, London: Arnold, p. 126.
A study of residential areas done by Hoyt (1939) in the North American context concluded that the land use pattern was not a random distribution, nor sharply defined rectangular areas or concentric circles, but rather sectors. Thus, the effect of direction and time was added to the effect of distance. Transport corridors, such as rail lines, public transit, and major roads, are mainly responsible for the creation of sectors. Transport has a directional effect on land uses with growth taking place along a major axis. The sector representation also includes concentric transitional processes observed by Burgess, which is occurring in a specific direction.
Following Hoyt’s development of a sectorial city, Harris and Ullman (1945) introduced a more effective generalization of urban land uses. It was brought forward that many towns and nearly all large cities do not grow around one CBD, but are formed by the progressive integration of a number of separate nuclei in the urban spatial structure. These nodes become specialized and differentiated in the growth process and are not located in relation to any distance attribute, but are bound by a number of factors:
- Differential accessibility. Some activities require specialized facilities such as port and rail terminals. For instance, the retailing sector demands maximum accessibility, which is often different from centrality offered in the CBD.
- Land use compatibility. Similar activities group together since proximity implies improved interactions through economies of agglomeration. Service activities such as banks, insurance companies, stores, and institutions are strongly interacting with each other. This can be defined as centripetal forces between activities.
- Land use incompatibility. Some activities are repelling each-other such as high quality residential and heavy industrial areas. This may be defined as centrifugal forces and one of the main reasons why poorer neighborhoods tend to be located on the eastern side, at least in industrial cities. Since in the northern hemisphere, prevailing winds tend to be westerlies, eastern sections of industrial cities tended to have a higher level of exposure to industrial air pollution.
- Location suitability. Some activities cannot afford the rent of the optimal site for their location. They are thus located at cheaper places, which are not optimal, but suitable for these activities.
Harris and Ullman poly-nuclear model was the first to represent the fragmentation of urban areas, specialized functions as well as suburbanization.