AP Human Geography Unit 6 Notes: Understanding Urban Form, Function, and Change

The Size and Distribution of Cities (Rank-Size, Primate City)

When geographers talk about a city’s size, they usually mean its population (sometimes its metropolitan population rather than just the city limits). When they talk about the distribution of cities, they mean how city sizes relate to one another within a country or region. This matters because city-size patterns often reflect a country’s history, level of development, colonial legacy, transportation networks, and how centralized political and economic power is.

A helpful way to think about it: imagine each city as a “node” in a national network of jobs, services, decision-making, and culture. Some networks are balanced, with several strong cities. Others are dominated by one huge city that functions like the country’s main control center.

The rank-size rule

The rank-size rule is a pattern that describes an “idealized” hierarchy of cities in a region: the second-largest city is about half the size of the largest, the third-largest is about one-third the size, and so on. It’s not a law of nature—it’s a model you use to compare real countries to a baseline.

A common way to express it is:

P_n = P_1 / n

• P_n = population of the city ranked number n
• P_1 = population of the largest city
• n = the city’s rank (2 for second-largest, 3 for third-largest, etc.)

Why it matters: If a country roughly follows the rank-size pattern, that often suggests a more integrated urban system—multiple cities can support high-level services (universities, corporate HQs, specialized hospitals) and economic opportunities may be more geographically spread out.

How it works (mechanism): In a balanced urban system, cities “compete” and “specialize” across regions. Transportation connections, regional economies, and migration flows help several cities grow rather than funneling growth into just one.

Show it in action (quick worked example):

• Suppose the largest city has 6 million people.
• Rank-size predicts:
  • 2nd city: about 3 million
  • 3rd city: about 2 million
  • 4th city: about 1.5 million

If the actual second city is only 800,000 while the largest is 6 million, the system is much more top-heavy than rank-size would predict.

What can go wrong (common misunderstanding): Students sometimes treat rank-size as a rule every country “should” follow. On the AP exam, it’s usually about interpretation: does a country match the pattern, and what might that imply about development, centralization, or history?

Primate cities

A primate city is a country’s leading city that is disproportionately large and dominates the country’s economy, politics, and culture—far beyond what you’d expect from rank-size. Often, the primate city is also the capital and the main gateway to the global economy.

Why it matters: Primate-city dominance can create strong national advantages (a concentrated hub for finance, universities, and innovation), but it can also deepen regional inequality. When the best jobs and services cluster in one city, internal migration may intensify, pushing rapid urban growth and housing pressures in that primate city while other regions lag behind.

How it happens: Several forces can produce primate cities:

• Colonial history: Colonial powers often developed a single port/capital for administration and export.
• Centralized political systems: Government offices and investment concentrate in one place.
• Transportation and trade: A major port or crossroads attracts firms and migrants.
• Agglomeration effects: Once many jobs and services cluster, the city becomes even more attractive.

Show it in action (conceptual example): If one city contains a very large share of a country’s population and most national institutions (major banks, government ministries, top universities), that city is functioning as primate—even if other cities exist.

What can go wrong (common misunderstanding): A primate city is not simply “the largest city.” It’s the largest by an outsized margin and usually plays a dominant national role.

Exam Focus

• Typical question patterns
  • Interpret a rank-size graph or table and decide whether the country follows rank-size or is primate-dominated.
  • Explain one cause and one consequence of primate-city dominance (often tied to colonialism, centralization, or uneven development).
  • Compare two countries’ urban hierarchies and connect patterns to development or governance.
• Common mistakes
  • Saying rank-size is “good” and primate is “bad” without explaining tradeoffs (efficiency vs. inequality).
  • Defining primate city as “largest city” without mentioning disproportionate dominance.
  • Ignoring scale: mixing up city proper vs. metro area when comparing sizes.

The Internal Structure of Cities (Models)

Cities aren’t random collections of buildings—they’re organized by land use (what different areas are used for), accessibility (how easy it is to reach places), land value, and historical development. Urban models simplify these patterns so you can predict where certain activities tend to locate.

A key idea underneath most models is that land closer to major nodes (like downtown) tends to be more valuable because it’s more accessible. Businesses that need maximum customer access often outbid other land users for central locations, while land uses that require more space (like single-family housing) often move outward.

Classic North American models

These models were developed to explain patterns in many U.S. cities during industrialization and early suburbanization. They are not perfect “maps” of every city; they are tools to reason about land use.

Concentric Zone Model (Burgess)

The Concentric Zone Model describes a city growing outward in rings from a central core.

How it works:

• The CBD (Central Business District) is at the center with high accessibility and high land values.
• Surrounding rings include zones of industry/transportation, older housing, and then newer commuter suburbs.
• The zone in transition (near the CBD) often includes aging housing and mixed land uses; historically, it was associated with recent immigrants and poverty as wealthier residents moved outward.

Why it matters: It highlights how invasion/succession and filtering can change neighborhoods over time.

Common misconception: Students sometimes think “poor always live closest to CBD.” In reality, many modern cities have wealthier downtown residents due to redevelopment and gentrification, and poverty can be suburban in some metros.

Sector Model (Hoyt)

The Sector Model argues cities develop in wedge-shaped sectors radiating out from the CBD, often along transportation routes.

How it works:

• High-income areas may develop along a pleasant corridor (scenic views, higher ground, away from industry).
• Industrial sectors often follow rail lines, ports, or highways.

Why it matters: It connects land use to transportation and environmental factors—cities don’t expand evenly.

Common misconception: Thinking sectors replace rings entirely. Many cities show both: ring-like gradients plus corridor development.

Multiple Nuclei Model (Harris and Ullman)

The Multiple Nuclei Model proposes that cities develop multiple centers (“nuclei”) instead of a single CBD.

How it works:

• Different activities cluster in different nodes: a downtown business core, a university district, an airport/industrial district, a shopping/office node, etc.
• Certain land uses attract or repel others (heavy industry may repel high-income housing; universities may attract apartments and bookstores).

Why it matters: This model fits modern metropolitan areas where jobs and services decentralize.

Common misconception: Believing the CBD disappears. Many cities still have an important downtown even with multiple nodes.

Peripheral (Galactic) Model / Edge cities

The Peripheral Model describes urban areas with an outer ring (often a beltway) and multiple business districts on the periphery. Edge cities are major suburban nodes of offices, retail, and entertainment that function like secondary CBDs.

How it works: Highways make peripheral locations highly accessible by car, encouraging firms to relocate to cheaper land with more parking and easier commutes for suburban workers.

Why it matters: It helps explain why commuting is no longer only “suburbs to downtown.” Many commutes are suburb-to-suburb.

Global urban models (often used in AP Human Geography)

Urban form differs across world regions because of colonial history, timing of industrialization, transportation, and income inequality.

Latin American City Model (Griffin-Ford)

This model emphasizes a strong CBD connected to an elite residential corridor.

Typical features:

• A central CBD with business and services.
• A spine of development (often a major boulevard) stretching from the CBD to high-income residential areas.
• Peripheral zones can include squatter settlements (informal housing) and areas with inadequate infrastructure.

Why it matters: It highlights how social inequality and historical development shape land use.

Southeast Asian City Model (McGee)

This model often shows a mix of colonial CBD influence and strong commercial activity outside the center.

Typical features:

• A central area shaped by colonial and global business.
• Significant commercial zones in other parts of the city (sometimes described as “multiple commercial nodes”).

Why it matters: It helps you avoid forcing U.S.-based models onto rapidly urbanizing regions with different histories.

African City Model (de Blij)

This model reflects a mix of traditional city centers, colonial influences, and modern CBD development.

Typical features:

• Multiple CBD-like areas (traditional market zones, colonial CBD, and modern business districts).
• Informal settlements may develop where housing demand outpaces formal supply.

Why it matters: It connects urban form to colonial history and contemporary economic change.

Comparing models (what to focus on)

Exam Focus

• Typical question patterns
  • Identify which model best fits a described city pattern (rings vs wedges vs multiple nodes).
  • Explain how transportation (rail, highways, ports) shaped a city’s internal structure.
  • Compare an MDC-style model to a regional model (Latin American, African, Southeast Asian) using inequality and colonial history.
• Common mistakes
  • Treating models as exact maps rather than simplified patterns used for reasoning.
  • Confusing “CBD” with any downtown-like area; some models include multiple business cores.
  • Overgeneralizing: assuming one regional model explains every city in that region.

Density and Land Use

Density is about how much “stuff” (people, housing units, floor space) is packed into a given area. Land use is about what that area is used for (housing, retail, offices, industry, parks, transportation). Together, density and land use help you explain why cities look and function the way they do—and why certain urban problems emerge.

A useful mental model is to connect three things:

1. Accessibility: How easy it is to reach jobs, customers, and services.
2. Land value: More accessible locations tend to cost more.
3. Land use intensity: Expensive land encourages vertical building and intensive use; cheap land allows spread-out uses.

Bid-rent and why land use clusters

Bid-rent theory describes how different land users (retail, offices, industrial, residential) are willing to “bid” different amounts for land based on how much they benefit from central accessibility.

How it works (step by step):

• Businesses that need high customer traffic (like premium retail) gain the most from central locations.
• They can afford higher rent because they earn more per unit of accessible location.
• Residential users usually can’t outbid top-tier commercial users in the most accessible spots, so housing tends to locate where land is cheaper.

This helps explain why the CBD tends to have high-rise offices, why land values often decline with distance from the CBD (not perfectly, but as a general trend), and why you might see specialized clusters (financial districts, warehouse districts).

Common misconception: Students sometimes think land value only depends on distance from the CBD. In reality, accessibility can be created by highway interchanges, transit hubs, waterfronts, airports, universities, and suburban job nodes—so you can get multiple “peaks” of land value.

Measuring density (what AP often expects)

AP questions typically use density conceptually (high vs low, increasing vs decreasing) rather than heavy calculation. Still, you should be clear about what density refers to.

• Population density: people per unit area.
• Residential density: housing units or people per residential land area (important because parks/industrial land can distort overall density).
• Floor-area ratio (FAR): a planning measure comparing building floor area to lot size; higher FAR generally means more intensive development.

Why it matters: Density shapes transportation choices and environmental impacts. Higher density can support frequent public transit and walkability; very low density often increases car dependence and infrastructure costs per person.

Land use patterns inside cities

Even without memorizing every zoning category, you should understand common urban land uses and why they locate where they do.

Central Business District (CBD)

The CBD is typically the most accessible point in the urban area and often contains:

• High-rise offices and corporate services
• Government buildings
• Major retail (though this has shifted in many cities)
• Transit hubs

Because land is expensive, buildings grow vertically and land use is intensive.

Industrial land use

Industrial areas often locate where transportation access is strong (rail lines, ports, highways) and where land is relatively cheaper than the CBD. Heavy industry historically clustered near rivers and rail; modern logistics may cluster near interstates and airports.

Residential patterns and segregation

Residential land use varies by:

• Income (ability to pay for space, location, amenities)
• Transportation (commute time and cost)
• Housing age and type (apartments vs single-family)
• Public policy (zoning, highway placement, public housing decisions)

It’s important to be cautious: urban models often describe patterns that emerged from specific historical conditions, including discriminatory housing practices in the U.S. (like redlining). On the exam, you may be asked to connect land use outcomes to policy and investment decisions, not just “personal preference.”

Suburbanization and land consumption

As cars and highways made commuting easier, many cities experienced suburbanization: population and jobs shifting outward. This often reduces average density and spreads land uses apart (separating housing from workplaces and shopping). That separation increases trip distances and tends to make driving the default.

Show it in action (two contrasting mini-scenarios):

• In a dense mixed-use area, a grocery store and apartments may be in the same neighborhood, making walking realistic.
• In a low-density single-use suburb, housing subdivisions may be separated from retail by major roads, making most trips require a car.

Zoning and planning (how rules shape land use)

Zoning is a legal tool that designates what can be built where (residential, commercial, industrial) and can regulate building height, lot size, parking, and density.

Why it matters: Zoning can protect residents from harmful land uses (like heavy industry next to homes), but it can also limit housing supply (for example, by requiring large lots or banning multi-family housing). محدود housing supply in high-demand cities often contributes to higher housing prices and can set the stage for gentrification pressures.

Exam Focus

• Typical question patterns
  • Explain why high land values in the CBD lead to vertical development and intensive land use.
  • Describe how transportation changes (highways, transit) alter density patterns and create multiple nodes.
  • Connect zoning or planning choices to housing availability, density, and urban form.
• Common mistakes
  • Confusing population density with residential density (overall density can be lowered by large industrial or park areas).
  • Claiming “density causes traffic” without noting that density can also support transit and shorter trips; the key is design and infrastructure.
  • Treating land use as purely market-driven and ignoring planning rules and public investment.

Urban Sustainability and Challenges (Sprawl, Gentrification)

Urban sustainability is about meeting current needs (housing, transportation, jobs, clean air and water) without making future conditions worse. Cities concentrate people and economic activity, which can be efficient—but rapid growth, unequal investment, and car-dependent design can produce major challenges.

Two major AP Human Geography themes in this section are urban sprawl and gentrification. Both involve land use, density, transportation, and inequality—so they connect directly to the models and density concepts you learned above.

Urban sprawl

Urban sprawl is the expansion of urban development outward over large areas, typically characterized by low-density, car-dependent, and often single-use zoning patterns.

Why sprawl happens

Sprawl is not just “people moving outward.” It usually results from a combination of:

• Transportation technology: Widespread car ownership and highway building reduce the time cost of distance.
• Cheaper land at the periphery: Housing developers can build larger homes on larger lots.
• Desire for space: Some households prefer larger yards or newer housing.
• Planning and zoning: Single-family-only zoning and minimum parking requirements can push development outward.
• Decentralization of jobs: Edge cities and suburban office parks reduce the need to live near downtown.

Why it matters (impacts)

Sprawl affects cities and regions in several interconnected ways:

• Environmental impacts: More land consumption can reduce habitat and farmland. Longer car trips increase emissions.
• Infrastructure costs: Roads, sewers, power lines, and schools must cover larger areas—often increasing per-capita costs.
• Public health and equity: Car dependence can disadvantage people who cannot drive (due to income, age, disability). It can also reduce opportunities for walking and increase exposure to air pollution near highways.
• Fragmented governance: Suburban growth often spans multiple municipalities, making coordinated planning harder.

Responses: smart growth and related strategies

Smart growth is a planning approach that aims to limit sprawl and make development more efficient and livable.

Common strategies include:

• Mixed-use development: Housing near shops and jobs.
• Transit-oriented development (TOD): Higher-density development near transit stations.
• Infill development: Building on vacant or underused land inside the existing urban area.
• Urban growth boundaries / greenbelts: Policies that limit outward expansion to protect open space.
• Complete streets: Designing roads for pedestrians, cyclists, transit, and cars (not just cars).

Common misconception: Students sometimes assume sprawl is only a “bad choice” by individuals. AP questions often reward you for recognizing the structural drivers—highways, zoning, and market incentives.

Gentrification

Gentrification is the process in which higher-income residents and investment move into a historically lower-income neighborhood, increasing property values and often changing the area’s culture and commercial landscape.

How gentrification works (step by step)

While every city is different, a typical process looks like this:

1. Disinvestment (often historical): Neighborhoods may have aging housing stock and lower property values, sometimes linked to discriminatory lending and unequal public investment.
2. Rent gap / investment opportunity: Developers and buyers see potential profit if the area is renovated and marketed to higher-income households.
3. In-migration of higher-income residents: People may be attracted by proximity to jobs, unique architecture, transit access, or new amenities.
4. Rising costs and changing land use: Rents rise, property taxes can increase, and local businesses may shift toward new consumers.
5. Displacement pressures: Long-time renters may be priced out; even homeowners can struggle with higher taxes and cost of living.

Why it matters (tradeoffs)

Gentrification is one of the most debated urban processes because it has both potential benefits and harms.

Potential benefits:

• Renovated housing and improved infrastructure
• Reduced vacancy and increased tax base
• New services and retail options

Potential harms:

• Displacement of long-time residents and cultural institutions
• Loss of affordable housing
• Social conflict over who the neighborhood is “for”

A strong AP-style explanation recognizes this complexity: gentrification is about reinvestment and neighborhood change, but also about power, housing markets, and equity.

Policies and strategies related to gentrification

Cities try various approaches to reduce harmful displacement while still encouraging neighborhood improvement, such as:

• Preserving or expanding affordable housing (including inclusionary zoning in some places)
• Tenant protections (varies widely by location)
• Community land trusts and nonprofit housing
• Targeted tax relief for vulnerable homeowners (policy-dependent)

Common misconception: Gentrification is not the same thing as “revitalization.” Revitalization is a broader term for improvement; gentrification specifically involves a class shift and rising costs that can displace residents.

Connecting sprawl and gentrification to urban structure

These two challenges are linked in a feedback loop:

• Sprawl can draw investment and higher-income households outward, leaving some inner-city neighborhoods with disinvestment.
• Later, renewed demand for central locations (shorter commutes, urban amenities) can drive reinvestment and gentrification.
• As central neighborhoods become more expensive, lower-income households may be pushed to cheaper suburban areas, changing the geography of poverty.

This is why AP Human Geography often treats urban issues as regional systems, not isolated neighborhoods.

Exam Focus

• Typical question patterns
  • Explain causes and consequences of suburbanization and sprawl, often tied to transportation and zoning.
  • Describe gentrification and analyze its effects on demographics, housing costs, and local businesses.
  • Propose or evaluate planning strategies (smart growth, TOD, infill) as solutions to sprawl or sustainability problems.
• Common mistakes
  • Defining gentrification only as “neighborhood improvement” without mentioning rising costs and potential displacement.
  • Treating sprawl as simply “city growth,” ignoring low-density, car dependence, and land consumption.
  • Listing solutions (like public transit) without explaining the mechanism (how it reduces car dependence or supports higher density).