We can classify permanent human populations into three categories:

• Rural areas (farms and villages) with low concentrations of people
• Urban areas (cities) with high concentrations of people
• Suburbs that are primarily residential areas near cities

Factors Driving Urbanization

A settlement is a place with a permanent human population. The first agricultural settlements appeared around 12,000 years ago. Before that, people survived by hunting and gathering, so they lived in temporary or movable shelters. The first permanent settlements were small enough that the inhabitants could all farm and subsist on the surrounding fields. Over time, in several places around the world, small agricultural settlements began to develop characteristics that made them the first true urban settlements, or cities:

• the presence of an agricultural surplus
• the rise of social stratification and a leadership class or urban elite
• the beginning of job specialization

A food surplus became available as irrigation, farming, and domestication of animals and plants developed. These changes enabled increasing numbers of people to live in the same location. A ruling class emerged to control the products that were accumulated and the people living in the community. Because not everyone was needed to produce food, some people specialized in making things, such as tools, weapons, and art. Others specialized as accountants or religious leaders-the first members of a service sector. As a result, cities developed as economic centers of services, manufacturing, and trade.

Urbanization

The process of developing towns and cities is known as urbanization, an ongoing process that does not end once a city is formed. Urbanization also involves the causes and effects of existing cities’ growth. Describing a region as urbanized indicates that cities are present there. A common statistic associated with regions, countries, and even continents is percent urban- an indicator of the proportion of the population that lives in cities and towns as compared to those that live in rural areas.

Urbanization is one of the most important phenomena of the 19th and 20th centuries, and geographers continue to study its development. Today, more than 50 percent of the world’s population lives in cities. Demographers estimate that by the year 2030, 60 percent will live in cities, and nearly 70 percent by 2050. Most of those people will be in the less-developed countries (LDCs) of the world’s periphery and semiperiphery. While urbanization can be positive for both individuals and societies, the challenges may be overwhelming if a city is not prepared to grow or if urbanization occurs too rapidly.

Influence of Site and Situation on Cities

The location of where cities develop is a critical aspect of urban geography. The concepts of site and situation, introduced in Topic 1.4, play a key role in explaining this process. Site describes the characteristics at the immediate location-for example, physical features, climate, labor force, and human structures. In contrast, situation refers to the location of a place relative to its surroundings and its connectivity to other places. Examples would include near a gold mine, on the coast, or by the railroad. Important factors of site and situation today are different than past cities.

The site and situation of a city influences its function. Specialized functions of cities include defense, religion, trade, education, finance, transportation, government, manufacturing, retirement, entertainment, residential housing, or service centers. Larger cities often have multiple functions. Cities near natural ports, such as Boston or New York City, started as centers of trade but provide multiple functions today.

Early City-States

Historically, a city-state consisted of an urban center (the city) and its surrounding territory and agricultural villages. A city-state had its own political system and functioned independently from other city-states. The population in the surrounding villages and territory received services and protection from the urban center. These communities were often raided by other groups for their wealth. As a result, defense was a primary consideration, and military leaders evolved into political rulers, or kings.

Early city-states emerged in several locations around the globe in an urban hearth, or area generally associated with defensible sites and river valleys in which seasonal floods and fertile soils allowed for an agricultural surplus:

• the Tigris-Euphrates Valley (Mesopotamia) in modern Iraq
• the Nile River Valley and Nile Delta in modern Egypt
• the Indus River Valley in modern Pakistan
• the Huang-He floodplain in modern China

Other urban centers also emerged in Mesoamerica (in modern Mexico) and in the Andean region of South America. Examples of city-states through history include those of Classical Greece (Athens, Sparta, Corinth), those of the Middle Ages in Europe, and Venice and Italian city-states during the Renaissance. Monaco, a city-state located entirely within the boundaries of Italy, has endured to modern times. Vatican City and Singapore are also modern city-states, though they did not evolve from previous agricultural settlements, but from religious influence. City-states eventually coalesced to form early states and empires. The ancient Babylonian Empire grew from the original city-state of Babylon.

Centers for Services

As cities grew, more people developed specialized skills other than producing food. This changed the relationship between cities and the areas around them. City residents depended on farmers for food. In return, people in cities focused on supplying services for their inhabitants and the inhabitants of surrounding regions.

Early cities often specialized in particular services. Some emerged as administrative centers from which the elite ruled. Others, often associated with important shrines, became religious centers. Defensive strongholds, university towns, and centers of specialized production -located at resource sites-also emerged.

Defining Cities

Most definitions of a city describe a place with a relatively high concentration of people. Cities are places where people come together to build a nucleated, or clustered, settlement. An urban area is usually defined as a central city plus land developed for commercial, industrial, or residential purposes, and includes the surrounding suburbs.

Legal Definition of a City

Definitions of what constitutes a city vary greatly, but the easiest way to define a city is a higher-density area with territory inside officially recognized political boundaries. This definition is useful for determining the precise population, taxing residents, providing services, and establishing and enforcing laws. Most large cities today, as defined legally, share boundaries with adjacent cities, yet those boundaries are visible only on a map. On the ground, people leaving one city might have no idea they were entering another legal city.

Metropolitan Areas

A collection of adjacent cities economically connected, across which population density is high and continuous is a metropolitan area, sometimes called a metro area. Most large cities in the world today are really metro areas of a series of legally defined cities, but they are referred to using only the name of the largest city. For example, the metro area of Denver, Colorado, consists of the cities of Denver, Aurora, Lakewood, Englewood, Greenwood Village, and other neighboring, legally defined cities.

In the United States, the term metropolitan statistical area (MSA) is another way to define a city. An MSA consists of a city of at least 50,000 people, the county in which it is located, and adjacent counties that have a high degree of social and economic integration, or connection, with the urban core. Similarly, micropolitan statistical areas are cities of more than 10,000 inhabitants (but less than 50,000), the county in which they are located, and surrounding counties with a high degree of integration. Note that this designation is really one in which a city is defined as a nodal region, or focal point in a matrix of connections.

Population Characteristics

People are drawn to urban areas often from rural areas, other regions, or other countries, looking for jobs and opportunities. For these reasons, social heterogeneity is particularly high in cities, meaning that the population of cities, as compared to other areas, contains a greater variety of people. Diversity in cultural interests, sexual orientations, languages spoken, professional pursuits, and other characteristics are present in cities to a much larger degree than in small towns or rural areas.

Immigration

One reason cities are diverse is because they are centers of immigration. For example, in several large cities around the world, 40 percent or more of the population is foreign born. Among these are Miami and San Jose in the United States, Toronto and Vancouver in Canada, and Sydney and Melbourne in Australia.

Diversity

Because of the higher population density and the relative anonymity of cities, urban residents are generally more accustomed to diversity than are people in non-urban areas. Walking through the streets of cities such as New York, London, or Amsterdam, one can see signs in numerous languages, restaurants that serve food from around the world, and buildings representing many religious traditions. Such diversity is less common in more sparsely populated areas.

One result of this diversity in cities is that it leads to more diversity. Cities have always attracted individuals with less common cultures, interests, or ways of life. They are more likely to find people whom they share traits with in cities.

Transportation and Communication

Improvements in transportation and communication have aided the growth of cities in size and number. Urban areas have expanded as trains, buses, and cars have enabled people to move farther from the center of the city, but still visit or work in the city. That change illustrates how time-space compression, in the form of transportation improvements, has led to urban growth. The development of the Internet-to transport ideas rather than people-has allowed more and more people to work from home, which has increased the distance people can live from the center of a city.

Borchert’s Transportation Model

Geographer John Borchert developed Borchert’s transportation model to describe urban growth based on transportation technology. Each new form of technology produced a new system that changed how people moved themselves and goods in and between urban areas. He divided urban history into four periods, which he called epochs. Each epoch had profound effects on the local scale related to a city’s form (shape), size, density, and spatial arrangement. Additionally, transportation had profound impacts on the distribution and connectivity of cities on a regional, national, and global scale.

While Borchert’s model ends in 1970, it could be expanded. Since 1970, some cities have encouraged mass transit (rail lines), biking (separate bike lanes on roads and new bike paths), and walking (car-free areas in cities). Additionally, the expansion and importance of jet air travel is not fully captured in the model.

Transportation’s Impact on Cities

Changes in transportation infrastructure within cities have also had important effects on the urban structure. The earliest urban centers were pedestrian cities, or cities shaped by the distances people could walk. A horse-and-buggy era allowed for city size to increase as people could move farther from the center and its concentration of services and jobs. Streetcar systems encouraged the movement of the population even farther from the center of a city, and growth became concentrated along the lines of these small urban rail systems. Streetcar suburbs, communities that grew up along rail lines, emerged, often creating a pinwheel-shaped city.

The advent of the automobile had profound effects on the growth of cities. Using cars and the highways built to facilitate movement, the population of cities spread out over ever-increasing distances from the urban core. The lower density suburbs that emerged around original cities developed as separate legal cities but functioned as part of the metropolitan area focused on the central, or original city. Additionally, with the U.S. interstate highway system, cities connected to highways have situational advantages of accessibility to road networks to transport goods or services more efficiently.

Today, major cities that hope to grow economically must have access to multiple modes of national and international transportation, as well as trade networks with air travel becoming increasingly important. Individual cities will often focus economic development policies to increase connectivity from the local to the global scale. Therefore, transportation methods have profound effects on the growth and shape of cities.

Communication Networks

Changes in communication technology have dramatic impacts on the growth and development of cities. Historically, cities connected to trade routes received information first. However, as telecommunication technology developed-with the telegraph, telephone, cell phones, and the Internet-early adopting cities benefitted.

Cities are nodal regions that require connectivity in order to thrive. New communication technologies diffused hierarchically to large cities first. In the early 2000s, cities that lagged in building new communication infrastructure fell behind cities that were on the cutting edge of technology, like Tokyo, Chicago, London, and New York City. Today, advanced communication networks are essential to attract large corporations, factories, or high-tech companies to an urban area in order to encourage further economic growth. In 2020, according to U.S. News World Report, the city with the best communication network (or smart city) was Singapore.

Population Growth and Migration

Rural-to-urban migration is an important concept to understand the growth of cities. Population growth pressure, cultural tension, environmental strain, and lack of economic opportunities create push factors in agricultural communities. Cities promise the hope of economic opportunities and cultural freedoms. Consequently, billions of people have migrated from agricultural regions to urban areas over the past 100 years. People are attracted to more densely populated cities to obtain higher-paying jobs and more government services.

Today, the most rapid rural-to-urban migration occurs in periphery and semiperiphery countries of the world, including China, India, and Brazil. The vast majority of this migration pattern occurs domestically, or within the country. For example, millions of people continue to migrate from rural northern and western Brazil to the rapidly expanding cities of Sao Paulo and Rio de Janeiro in southeast Brazil. This rapid growth has stretched resources and created challenges for cities, such as substandard housing, overcrowding, and stressed infrastructure (transportation, sanitation, and water systems).

In core countries, such as the United States, rural-to-urban migration has slowed but domestic and international migration to cities in the west and south has increased. The increase is due to the perception of economic opportunity, cost of living, and quality of life in those regions. Urban areas, such as Atlanta, Tampa, and Charlotte in the South; and Denver, Dallas, and Phoenix, in the West, have experienced rapid urban population growth that has created challenges for local communities, governments, and residents.

Economic Development and Government Policies

Increasingly, cities are viewed as engines of growth for a country’s economy. Consequently, economic and political leaders, at the national and local scale, develop policies to guide and encourage the growth of cities. Cities can have a variety of different functions and economic emphases. Cities in the Midwest of the United States, such as Cleveland, Pittsburgh, and Chicago, were often focused on attracting manufacturing jobs; while cities in Florida, such as Orlando and Tampa, promoted development based on retirement and tourism. Local policies that created economic incentives, such as low-cost loans, lower taxes, or cheap available land, were used to encourage economic development. However, the economic function of a city can change over time. Today, Pittsburgh has changed its economic development policies to attract high-tech industries, such as Google and Uber, to take advantage of the highly educated students graduating from Carnegie Mellon University and the University of Pittsburgh. Within the high-income countries of the world, cities often compete with each other to attract companies and jobs.

Policies at the national scale can also impact the growth and development of cities. In 2014, China implemented the New Urbanization Plan that developed specialized cities and designated the eastern coast of the country for urbanization. The plan used a variety of methods such as tax incentives, land grants, and the creation of a series of connected cities, each with an economic focus. An example would be Shenzhen, a city near Hong Kong where the Chinese government has invested heavily in higher education and the high-tech industry to create a global financial center and its own Silicon Valley.

The Size and Distribution of Cities

Today, cities range in size from just a few thousand inhabitants to those that have populations of over 20 million, such as Karachi, Pakistan. Often a city exists in an urban system-an interdependent set of cities that interact on the regional, national, and global scale. Models have been developed to help explain the distribution, or location, and interaction of these urban systems.

Urban Hierarchy

Systems of cities have an urban hierarchy or ranking based on influence or population size. On the global scale, world cities are at the top of the hierarchy regarding influence or power. Megacities and metacities are on the top of the urban hierarchy when considering population. On a national or regional scale, the concepts of rank-size rule and primate city are utilized to determine the hierarchy within a country’s urban system.

Rank-Size Rule

The rank-size rule describes one way in which the sizes of cities within a region may develop. It states that the n’ largest city in any region will be 1/n the size of the largest city. That is, that the rank of a city within an urban system will predict the size of the city. For example, the third-largest city in a system that exhibits the rank-size distribution would be approximately one-third the size of the largest city.

Geographers consider rank-size distributions to be characteristic of well-developed regions or countries. Such distributions are also more common where federal governments typically share power with other levels of government. A rank-size distribution includes cities of all sizes in the system. This implies that there are cities with a wide variety of services available within the system, from very high-order services in the largest cities to lower-order services in the smaller cities. Higher-order services are usually expensive, need a large number of people to support, and are only occasionally utilized. Examples include major sports teams, large malls, luxury car dealerships, and large specialized research hospitals. Lower-order services are usually less expensive than higher-order services, require a small population to support, and are used on a daily or weekly basis. Examples include gas stations, local grocery stores, or small restaurants.

As a general rule, geographers consider rank-size distribution to be an indicator of an urban system that can efficiently provide needed services to its population. Countries that demonstrate the rank-size rule include the United States, Canada, Australia, and India. The model is not exact and applies better to some countries than others. Limitations of the model are that it does not explain the distribution of cities nor does it take into account the distance or interactions between cities. Problems can arise when comparing city systems in multiple countries because places define cities differently. In general, the model works better when using metropolitan area population.

Primate Cities

If the largest city in an urban system is more than twice as large as the next largest city, the largest city is said to have primacy, or be a primate city. A primate city is more developed than other cities in the system, and consequently, disproportionately more powerful. Primate cities are the social, political, and economic hub for the system and offer a wider range of services than do the many smaller cities. In primate city urban systems, medium-sized cities are often not present. In addition, countries that follow a unitary form of government, or extremely strong central government, often follow a primate city model.

The United Kingdom exhibits urban primacy. London is by far the largest city in the country. However, the relatively small size of the country, its unitary government, and its well-developed transportation infrastructure, all reduce the need for a number of medium-sized cities. In the United Kingdom, people can get to London for higher-order services relatively easily. Northern Scotland is less than a two-hour flight from London.

Mexico illustrates a different model for a country with a primate city. Mexico City provides many services that are not as easily available to portions of the population. Across large portions of northern Mexico, people would have to travel great distances to receive even mid-level services due to the lack of medium-sized cities. Because of Mexico City’s primacy, people often migrate to the city in search of economic opportunity and greater services.

Gravity Model Interactions

The gravity model states that larger and closer places will have more interactions than places that are smaller and farther from each other. This model can be used to predict the flow of workers, shoppers, vacationers, migrants, information, mail, products, economic activity, and nearly any other flow between cities. The model holds that there are more numerous flows to bigger cities and between nearer cities. Assumptions of this model include locations with no barriers, which is also considered a limitation. Other limitations with the model include not accounting for how political (borders), physical (walls or rivers), or cultural (language) barriers influence the interactions between cities.

Interactions between cities are complicated by factors beyond size and distance. Cities such as Orlando, Florida, and Las Vegas, Nevada, are tourist destinations that attract far more visitors than their size and their distance from other cities alone could predict. Similarly, religious sites such as Jerusalem and Mecca, government centers such as Washington, DC, and various cultural destinations distort effects predicted by the gravity model. However, the basic theory applies to most places.

Central Place Theory

In 1933, Walter Christaller, a German geographer, proposed the central place theory to explain the distribution of cities of different sizes across a region. The model used consumer behavior related to purchasing goods and services to explain the distribution of settlements. Christaller defined a central place as a location where people go to receive goods and services. It might be a tiny community, such as a hamlet, with only lower-order services, such as a convenience store, post office, and religious center. Or it might be a slightly larger village, town, or small city with more stores and services. Or the central place might be a major city, where one can get lower- and higher-order services, such as direct air flights to other major cities or watch a touring Broadway musical. In Christaller’s model, each size of settlement would be evenly distributed across space.

The model accurately concludes that larger cities will be farther spaced from each other than smaller towns or villages. This conclusion is evident across multiple locations but particularly when viewing the distribution of cities in the eastern United States. (See North America at night map, page 5.) Large cities like Chicago and Atlanta have a series of medium cities between them that are roughly the same distance from each other.

The Shape of Market Areas

A market area, or zone that contains people who will purchase goods or services, surrounds each central place. Higher-order services have larger market areas than lower-order services. Christaller chose to depict these market areas as hexagonal hinterlands because this shape was a compromise between a square-in which people living in the corners would be farther from the central place-and a circle-in which there would be overlapping areas of service. Nesting hexagons allowed for central places of different sizes to distribute themselves in a clean pattern across the region.

Threshold and Range

What determines which services will be available in any central place? How far apart should central areas of the same population size be located? Central place theory uses the concepts of threshold and range to answer these questions.

The size of population necessary for any particular service to exist and remain profitable is the threshold. Services with a very low threshold, such as a convenience store or a gas station, are present even in very small central places. Restaurants, hospitals, high schools, and department stores have higher thresholds, so they require a larger population within the market area to survive economically. Only in the largest market areas can services appear that depend on the support of huge populations-stock market exchanges, major sports teams, symphony orchestras, and elite research centers. As cities grow in size, the number and variety of available services increase with the population.

The distance people will travel to obtain specific goods or services is range. People will travel very far for higher-order services such as wedding rings and heart transplants, but they are less likely to travel very far for basic services such as fast food or toothpaste. This helps explain why fast-food restaurants can be found in nearly any town but a shop dealing in diamond jewelry would be found only in larger cities.

Limitation of Central Place Theory

A limitation of the model is that it assumes a flat, featureless plain. It does not take into account the effects of natural landscapes of rivers, mountains, or other barriers on the distribution of cities. Nor does it consider the influence of transportation systems (rail, road, water, and air) and how the availability of those types of transportation can expand the market area.