Topic 6.4

Essential Knowledge

Geographers use several models and principles to explain why cities are different sizes, how they are distributed, and how people interact with them. These models help identify patterns rather than predict exact outcomes. 

Rank-Size Rule 

The rank-size rule states that in a well-developed urban system, the population of a city is inversely proportional to its rank in the urban hierarchy. 

• The largest city is ranked first 
• The second-largest city should have about ½ the population of the largest 
• The third-largest city should have about ⅓ the population, and so on 

Countries that follow the rank-size rule tend to have: 

• A balanced distribution of cities 
• Multiple large and mid-sized cities 
• Strong economic integration across regions 

This pattern is most common in more developed countries with long histories of urban growth and strong transportation networks. 

Primate City Rule 

A primate city is a city that is at least twice as large as the second-largest city and dominates the country economically, politically, and culturally. 

Characteristics of primate city systems include: 

• Extreme concentration of population and resources in one city 
• Limited development of secondary cities 
• Strong pull of migrants toward the dominant city 

Primate cities are more common in: 

• Developing countries 
• Countries with colonial histories 
• Nations with centralized political systems 

While primate cities can drive economic growth, they often contribute to regional inequality, congestion, and strain on infrastructure. 

Gravity Model 

The gravity model is used to predict interaction between two cities based on: 

• The population size of each city 
• The distance between them 

According to the model: 

• Larger cities attract more people, goods, and services 
• Cities closer together interact more than cities far apart 

The gravity model helps explain: 

• Migration patterns 
• Retail trade areas 
• Transportation flows 
• Communication and travel frequency 

Although it does not account for cultural or political barriers, the gravity model is useful for understanding spatial interaction between places. 

Central Place Theory 

Central place theory explains how cities and towns are spatially distributed to provide goods and services to surrounding areas. 

Key concepts include: 

• Central places: settlements that provide services 
• Market area (hinterland): the region served by a city 
• Threshold: the minimum population needed to support a service 
• Range: the maximum distance people are willing to travel for a service 

According to the theory: 

• Small settlements provide low-order goods (everyday items) 
• Large cities provide high-order goods (specialized or expensive items) 
• Settlements are arranged in a hexagonal pattern to minimize overlap and maximize efficiency 

Central place theory assumes a flat, evenly populated surface, making it an idealized model, but it remains useful for understanding retail and service patterns. 
 

Big Picture Takeaway 

Models such as the rank-size rule, primate city rule, gravity model, and central place theory help geographers explain patterns of urban distribution, city size, and spatial interaction. While no model perfectly describes the real world, each provides a framework for analyzing how cities develop and function within larger systems. 

Contextualization

Video #1

In Practice

Essential Question  

Identify the different urban concepts such as hierarchy, interdependence, relative, size, and spacing that are useful for explaining the distribution, size, and interaction of cities.  

Conceptual Analysis 

1. How might a country’s history of colonialism influence whether its urban system more closely resembles the primate city rule or the rank-size rule?
2. Many developing countries have one dominant city that attracts migrants from rural areas. Explain how this pattern can create both opportunities and challenges for national development.
3. How do urban hierarchy models help governments decide where to invest in transportation, hospitals, universities, or other public services?

Vocab Review 

Provide a shorthand working definition for each of the following terms. This means it needs to be in YOUR OWN WORDS. 

1. Primate City 
2. Gravity Model 
3. Christaller’s Central Place Theory 

MCQ

(You only need to write the letter of the answer that you choose)

1. A country whose second-largest city has approximately half the population of its largest city most closely follows which principle? 

A. Primate city rule 
B. Gravity model 
C. Central place theory 
D. Rank-size rule 
E. Urban hierarchy inversion 

2. Which urban pattern is most likely to be found in a country with a primate city? 

A. Even distribution of political and economic power 
B. Strong development of multiple mid-sized cities 
C. Concentration of infrastructure and services in one dominant city 
D. Limited rural-to-urban migration 
E. Equal access to services across regions 

3. According to the gravity model, interaction between two cities is greatest when the cities: 

A. Have similar cultural characteristics 
B. Are politically aligned 
C. Are large and located close together 
D. Specialize in different economic activities 
E. Have evenly distributed populations 

4. Which service is most likely to have the largest threshold and range according to central place theory? 

A. Gas station 
B. Grocery store 
C. Hair salon 
D. Automobile dealership 
E. Convenience store 

5. Central place theory best helps explain which of the following patterns? 

A. Why cities experience gentrification 
B. Why megacities grow rapidly in developing countries 
C. Why retail services are spaced unevenly across regions 
D. Why some cities dominate global finance 
E. Why urban sprawl occurs