Cities and Urban Land-Use Patterns and Processes

A city's site is its physical setting, such as a harbor, river crossing or easily defended hill, and its situation is its location relative to other places, such as trade routes and nearby cities. Cities grow through migration and natural increase, and changes in farming, industry, transportation and communication have each pushed urbanization forward.

Most fast urban growth today is in Asia and Africa, home to many megacities (10 million or more people) and metacities (20 million or more); by the UN's 2025 estimates, which use a new way of drawing city limits, Jakarta, Dhaka and Tokyo are the largest. In the United States and other higher-income countries, growth has spread outward through suburbanization and sprawl, creating edge cities (new office and shopping hubs at the edge of a metro), exurbs beyond the suburbs, and boomburbs (fast-growing suburban cities of 100,000 or more).

World cities (also called global cities), such as New York, London and Tokyo, are command centers of the global economy, with major company headquarters, banks, stock exchanges and media. Cities form an urban hierarchy, and new ideas, products and trends often spread from world cities down to smaller cities, an example of hierarchical diffusion.

The rank-size rule says a country's second-largest city has about half the population of the largest, the third about a third, and so on; a primate city breaks that pattern by being more than twice as large as the next city, like Bangkok in Thailand. Christaller's central place theory explains the spacing of towns by how far people will travel for a service (range) and how many customers it needs (threshold), and the gravity model predicts more interaction between places that are bigger and closer together. Like most models, these assume ideal conditions, such as flat land and evenly spread people, so real city systems fit them only roughly.

Urban models try to show how land is used inside cities: Burgess's concentric zone model (drawn from 1920s Chicago), Hoyt's sector model, Harris and Ullman's multiple nuclei model, the galactic city model of spread-out U.S. metros, and models of Latin American, Southeast Asian and African cities. Each simplifies one region and era, so use them to compare and explain real cities, not as exact maps.

Housing density ranges from low (detached single-family homes on their own lots) to medium (townhouses and duplexes) to high (apartment buildings). Zoning rules, land prices and transportation shape how dense a neighborhood becomes, and infilling, building on empty or underused lots inside the city, adds homes without spreading outward.

Infrastructure is the shared systems a city runs on, such as roads, public transit, water and sewer lines, power grids and internet service. Where it is built affects where businesses locate, which neighborhoods grow and how easily people can reach jobs, so gaps in infrastructure can hold an area back.

Planners try to make cities more livable and sustainable with mixed-use development, walkable streets, transit-oriented development, smart growth, New Urbanism, greenbelts and slow-growth policies. These ideas can cut car trips and protect open land, but critics point out that they can raise housing costs and sometimes push lower-income residents out.

Geographers study cities with quantitative data, such as census counts and surveys of income, age or housing, and with qualitative data, such as field observations, interviews and residents' own stories. Each kind reveals different things, so combining them gives a fuller picture of how a neighborhood is changing and what it needs.

Many urban problems trace back to discriminatory practices. Starting in the 1930s, redlining denied or limited home loans in neighborhoods with Black and immigrant residents, and in blockbusting, real estate agents stirred up racial fears so white owners would sell cheaply, then resold the homes for more. Cities today deal with housing affordability, environmental injustice, disamenity zones and informal (squatter) settlements. Urban renewal and gentrification can bring investment but also displace longtime residents, while inclusionary zoning requires new housing projects to include affordable homes.

Growing cities face sprawl, gaps in sanitation, climate risks such as flooding and extreme heat, and a large ecological footprint, meaning the land and resources needed to support their residents. Responses include urban growth boundaries, cleaning up and reusing brownfields (old, polluted industrial sites), protecting nearby farmland and adding green space, and people still debate how well each one works.