Cities are enormously complex and important centers for much of the world’s population. Since cities first emerged in human civilization, they have been centers of economic, political, and cultural power. They have been places of innovation. Cities are growing faster today than ever before in history and that trend is projected to continue.

Urban Models

Like most other models used by geographers, urban models are based on observations of real places. Though models vary, all models share certain functions:

• classifying and categorizing land use in urban areas
• describing how various urban land uses are segregated spatially
• offering explanations for the location of different urban land uses

Urban Zones

One principle underlying all urban models is functional zonation, the idea that portions of an urban area—regions, or zones, within the city—have specific and distinct purposes. The various zones fit together like a puzzle to create the entirety of the city. However, unlike a puzzle, the pieces of a city are not clearly delineated, and geographers have tried to identify and classify them with models. The resulting urban models provide geographers with a framework to describe, understand, and analyze cities. Urban areas around the world share three basic zones: the central business district, industrial/commercial, and residential.

Central Business District

A vital part of any urban model is the central business district (CBD), which is the commercial heart of a city. Often located near the physical center of a city, or the crossroads where the city was founded, the CBD is the focus of transportation and services. The bid-rent theory explains agricultural land use, just as it helps explain land use in central business districts. This theory explains that land in the center of a city will have higher value than land farther away from the city’s center. Therefore, land use will be more intense and costs will be higher closer to the CBD. This means high-order services often dominate the CBD.

Competition for valuable space in the CBD gives it certain characteristics:

• In some countries, including the United States and Canada, the CBD has skyscrapers and “underground cities” that might include facilities for parking, shopping, and rapid transit.
• In Europe, many CBDs are located in the historic heart of the city where buildings are shorter but services are still concentrated.
• Because the cost of land is high in CBDs, manufacturing activities are rarely located there.
• High costs and limited space often result in residential portions of CBDs having high-density housing, such as high-rise apartment buildings.

Industrial/Commercial Zone

The zone outside the central business district is dedicated to industry. These industrial zones may include manufacturing, warehousing, and transportation. Industrial zones are generally separated from residential areas because they are associated with air and noise pollution.

Commercial areas with lower-order services and less-intensive land use are also found outside the CBD. Law firms might locate in the CBD, but department stores usually prefer commercial shopping zones with land values.

Several factors influence the choice of locations for businesses within the commercial zone. First, the land is zoned for commercial use so they are legally allowed there. Second, some industries have a commensal relationship, which is when commercial interests benefit each other. For example, restaurants and theaters benefit by being in the same zone, as do clothing stores and shoe stores.

Residential Zone

All cities have residential zones, areas where people live. These are generally separate from the CBD and industrial zones either legally—through government zoning—or simply by the choices of inhabitants.

The different residential zones are distinct from one another. They may be segregated by density, income level, ethnic group, religion and culture, social status, or other characteristics. Which characteristic distinguishes the residential zones depends on the world region where the city is located.

Models of North American Cities

Three models describe typical urban areas in North America—the concentric zone model, sector model, and multiple-nuclei model. These “classic models” were based on the city of Chicago. It was a good place to examine urban structure without the complications caused by irregular topography.

Concentric Zones

The concentric zone model describes a city as a series of rings that surrounds a central business district. It is known as the Burgess model because sociologist E. W. Burgess proposed it in the 1920s. The first ring surrounding the CBD is a transition zone that mixes industrial uses with low-cost housing. Manufacturing benefits from proximity to the city-center workers and affordable land. Housing in this zone is often high-density, consisting of older, subdivided homes.

The next three rings are residential. Moving outward, one is for working-class housing, then one of more expensive housing, and finally, one of larger homes on the edge of the city and in the suburbs. With greater distance from the CBD, land is more plentiful and affordable, so residences are larger and of higher quality, and population densities decrease.

Sectors

In the 1930s, economist Homer Hoyt developed the sector model, also called Hoyt’s model. While Burgess used land-use rings that grew outward from the CBD, Hoyt described how different types of land use and housing were all located near the CBD early in a city’s history. Each grew outward as the city expanded, creating wedges, or sectors of land use, rather than rings.

Hoyt’s model describes sectors of land use for low-, medium-, and high-income housing. The model locates the sectors for the low-income, lower-quality housing next to these industrial and transportation zones, and it places high-income residences extending in a wedge away from these zones

along wide tree-lined boulevards or on higher ground. The model also notes a sector for transportation extending from the city’s center to the edge. This sector would contain rail, canal, and other transport networks within it. The transportation sector would also favor an adjacent zone of manufacturing.

Multiple Nuclei

Geographers Chauncy Harris and Edward Ullman developed the Harris and Ullman multiple-nuclei model by studying changes in cities in the 1940s. This model suggested that functional zonation occurred around multiple centers, or nodes. The characteristics of each node either attracted or repelled certain types of activities. The result was a city that consisted of a patchwork of land uses, each with its own center, or nucleus. In the multiple-nuclei model, the CBD and related functions continued to exist but were joined by smaller business districts that emerged in the suburbs. A zone of industry could be in a variety of locations, including the traditional CBD or port, or it could move to new outlying locations near an airport or other transportation junction. This industrial zone would attract related industries and an area of higher density housing. A university or a business park might attract nearby restaurants, theaters, and other amenities. As a result, people might create a district of student housing or high-quality homes nearby.

The peripheral model, a variant of the multiple-nuclei model, describes suburban neighborhoods surrounding an inner city and served by nodes of commercial activity along a ring road or beltway. This model’s name derives from the role of the service nodes with the related suburbs that develop on the periphery of the original city.

Galactic Cities

Beginning in the 1950s, suburban growth in the United States skyrocketed as governments built highways that improved transportation in and out of cities and subsidized home purchase. Based on this process in Detroit, Chauncy Harris developed the galactic city model. In it, an original CBD became surrounded by a system of smaller nodes that mimicked its function. As suburbs grew, they took on some CBD functions. At key locations along transportation routes, people created mini-downtowns of hotels, malls, restaurants, and office complexes. Some of these nodes grew large enough to become edge cities (see Topic 6.2), but they left behind a declining inner city.

World-Regional Models

Geographers have also developed models to describe cities outside of North America. Rings, sectors, and multiple-nuclei are found in these models, along with some additional elements. But the models share the same basic characteristic of North American models, that of functional zonation.

European Cities

Many of today’s cities in Europe grew out of medieval and pre-industrial cities. City walls, which were built for protection before the wide use of gunpowder weapons, restrained growth. These cities grew slowly and with little planning for centuries. The result is now a dense mix of commercial and residential land use with narrow, winding streets. Distinct land-use zones are difficult to find in the core areas of these cities. Later urban renovations cut through areas to produce elegant, wide boulevards with high-quality housing and shops.

CBDs in Europe differ in important ways from those in North America. Attempting to preserve their historic urban cores, city leaders have limited new construction and restricted the height of buildings. Often, former palace grounds have become large urban parks. European CBDs also have many more aerial view of Paris with Eiffel Tower and skyscrapers of La Defense. Paris is a planned city with the historical districts in the center of the city. The newer skyscraper business district is outside the city center.

Middle Eastern and Islamic Cities

The spread of Islam shaped many cities in the Middle East, North Africa, and parts of Spain, East Africa, and Southeast Asia. Dominating these cities is a central mosque that includes one or more tall and highly visible minarets, or tall slender towers. The principal mosque in the center of an Islamic city is usually surrounded by a complex of structures to serve the public, such as schools and soup kitchens. As cities grew, additional mosques were added in outlying neighborhoods.

Many Islamic cities were built with a defensive citadel, a fort designed to protect the city, with its related palace and barracks for soldiers. Walls with gates and towers were typical in earlier times and they, or their remnants, still survive in many modern Islamic cities. Major roads connect the gates of the citadel to the city center. Along these roads are traditional outdoor markets or covered bazaars, called suqs. These markets often exhibit spatial differentiation with shops selling luxury items near the center of the city, with bulkier, less-valuable materials for sale near the wall and gates.

Residential neighborhoods often reflect differences in ethnicity or branch of Islam in their organization and architecture:

• Streets and alleys are usually twisting and often dead-end.
• Homes have central courtyards rather than yards in front or back.
• Windows are small and located above eye level.

The above features create shady areas, which suggests they might be cultural adaptations to the sun and heat of the Middle East. These features also imply that privacy is an important value within Islam.

Latin American Cities

The Griffin-Ford model is often used to describe Latin American cities. It places a two-part CBD at the center of the city—a traditional market center adjacent to a modern high-rise center. The most desirable housing in the city is located there, next to the developed center of the city. This high-quality housing extends outward from the urban core, accompanied by a commercial spine of development. Theaters, restaurants, parks, and other amenities are also located along this spine, or corridor. The spine ends in a growing secondary center, also called a mall.

In contrast to the concentric zone model in North America, as distance increases from the center of Latin American cities, the quality of housing decreases. Public transportation, the urban water supply, and access to electricity all decrease farther away from the center, sometimes disappearing altogether. Often, Latin American cities have a zone of in situ accretion that acts as a transitional area between the older areas of the central city and the peripheral outer ring. The outer ring of the city, the periférico, shows poverty, lack of infrastructure, and areas of poorly built housing known as shantytowns. Often, the residents of shantytowns are recent migrants to the city. The model notes the possible presence of an industrial node closer to the commercial spine.

Many Latin American cities include favelas, or barrios, which are neighborhoods marked by extreme poverty, homelessness, and lawlessness. Most favelas are in disamenity zones, areas not connected to city services and under the control of criminals. They are often in physically unsafe locations, such as on steep, unstable mountain slopes. Structures are poorly constructed, often by the residents themselves, and densely packed together.

African Cities

Large cities were rare in most of Africa until the 19th century when Europeans colonized the continent. But in recent decades, urban areas in Africa have grown rapidly. New cities have been built next to or on top of existing ones. These new cities can include several identifiable regions:

• The Traditional CBD: This area existed before European colonization and has small shops clustered along narrow, twisting streets. It includes the formal economy with permanent stores that offer full-time jobs, comply with local regulations, and have set wages.
• The Colonial CBD: This area has broad, straight avenues and large homes, often with expansive lawns. It reflects the influence of European colonial powers on the city’s development.
• The Informal Economy Zone: This includes the vibrant street markets and informal businesses that operate without official licenses or permits. This zone is characterized by flexibility and entrepreneurship but lacks formal job security and regulatory compliance.
• Residential Zones Based on Ethnicity or Nationality: These neighborhoods are often segregated by ethnic or national groups, reflecting the diverse makeup of the urban population. They can vary greatly in terms of housing quality and access to services.

Southeast Asian Cities

The McGee model describes the land use of many large cities in Southeast Asia, where the focus of the modern city is often a former colonial port zone. This export-oriented zone shares commercial uses similar to the CBD in North American cities. Additionally, these cities might include a government zone. If the city is a national or regional capital, it might have a commercial zone dominated by foreign merchants and ambassadors. A belt of market gardening often surrounds and supplies these cities.

Cities in Southeast Asia have a history of Chinese immigration and commercial interest that dates back a few centuries. As a result of this immigration, many cities include a secondary commercial zone dominated by Chinese businesses. As the importance of industry in Southeast Asia has risen in the last few decades, industrial parks and regions of manufacturing have emerged on the peripheries of some cities.