Cities are weird. We spend our lives weaving through them, stuck in their traffic, and paying way too much for their tiny apartments, yet we rarely stop to ask why the Starbucks is where it is or why the "bad" part of town stayed that way for sixty years. That’s basically the heart of AP Human Geography Unit 6. It isn't just a list of vocabulary words about skyscrapers; it’s the study of the spatial logic—or sometimes the lack thereof—behind where humans huddle together.
Honestly, most students trip up here because they try to memorize the Burgess Model like it's a static map. It’s not. It’s a snapshot of 1920s Chicago that looks almost nothing like a modern sprawling metropolis in the Sun Belt. If you're looking at a city today, you're looking at a messy, layered history of redlining, transit shifts, and "edge cities" that popped up because nobody wanted to drive all the way downtown anymore.
Why the Site and Situation of AP Human Geography Unit 6 Matters
When we talk about cities, geographers start with two concepts: site and situation. Site is the physical dirt. Is it on a hill? Is it a swamp like D.C.? Situation is about the "vibes" and the connections. Singapore has a decent site, but its situation—sitting right on one of the world's busiest shipping lanes—is why it's incredibly wealthy.
Think about why New York became New York. It wasn't just the harbor. It was the Erie Canal. That single piece of infrastructure changed the "situation" of a small island, connecting it to the massive resources of the American Midwest. Suddenly, it wasn't just a coastal town; it was the gateway to a continent.
Urbanization is moving fast. Like, really fast. In 1950, only about 30% of the world lived in cities. Now? We're well over 50%, and by 2050, it’ll be closer to 70%. This massive shift creates "megacities" like Lagos or Tokyo, where the infrastructure is constantly gasping to keep up with the population. You’ve got people moving for "pull factors"—better jobs, flashy lights, the hope of a promotion—but often they're fleeing "push factors" like failing crops or conflict in rural areas.
The Models are Old, but the Logic Sticks
You'll see the Burgess Concentric Zone Model. It looks like a target. The idea is that the closer you are to the Center Business District (CBD), the higher the land value. This is "bid-rent theory" in action. Basically, the guy selling diamonds wants to be in the center where everyone sees him, so he pays more for a tiny shop. The farmer needs tons of land, so he stays way out where it’s cheap.
Then you have the Hoyt Sector Model. Hoyt realized that cities don't grow in perfect circles because we have things like trains and highways. If a factory line goes north, the "low-class" housing (worker housing) follows it because of the noise and smog. The wealthy people? They move as far away from the smoke as possible, creating a "high-class" sector on the opposite side.
👉 See also: The Honest Truth About What to Get a Good Friend for Christmas
But look at a city like Los Angeles. It doesn’t have one "center." It’s a mess of many centers. This is the Multiple Nuclei Model. It’s what happens when everyone has a car. You don’t need to go to the "main" downtown to buy a suit or see a doctor. You go to a suburban node. Harris and Ullman caught onto this in 1945, and it’s basically the blueprint for the American "galactic city" or edge city phenomenon we see today, where the beltway is more important than the city hall.
The Global South is Different
Don't apply the Chicago model to Sao Paulo. It won't work. In the Griffin-Ford Model (Latin American City Model), the "best" housing is actually in the center and along a "spine" leading out to a secondary mall. The further you get from the center, the more the infrastructure crumbles. This is the opposite of the "suburban flight" we see in North America.
In African cities, you often see three CBDs. You might have a colonial CBD with straight streets and old government buildings, a traditional market CBD where the real commerce happens, and a transitional business zone. It’s a literal map of history.
The Dark Side: Gentrification and Redlining
We can't talk about AP Human Geography Unit 6 without getting into the stuff that actually hurts. Urban geography isn't just about where buildings are; it's about who is allowed to live in them. For decades, the US government practiced "redlining." They literally drew red lines on maps around Black neighborhoods and told banks, "Don't lend money here."
The result? Total disinvestment. Houses fell apart because nobody could get a home improvement loan. Businesses left. This created the "food deserts" and "poverty traps" we still see today.
Now, we see the pendulum swing back with gentrification. Young professionals move into these "undervalued" areas because they want to be close to work. Property values go up. That sounds good, right? Not if you're the grandma who has lived there for 40 years and can no longer afford the property taxes. The neighborhood gets a cool new coffee shop, but the people who built the community get pushed out to the suburbs—which, ironically, are now becoming the new zones of poverty in many states.
Megacities and the Struggle for Sustainability
A "megacity" has over 10 million people. A "metacity" has over 20 million. Places like Mexico City, Manila, and Seoul are massive. The problem is "urban sprawl." We just keep building outward. It’s terrible for the environment. You have to pipe in water from hundreds of miles away, and everyone has to drive two hours to get anywhere.
This is where "New Urbanism" comes in. The goal is "walkability." Imagine a neighborhood where your grocery store, your gym, and your job are all within a 15-minute walk. It reduces the carbon footprint and, honestly, makes people less miserable. But it’s hard to do because our current laws (zoning) often forbid putting a shop in the middle of a residential neighborhood.
Cities also create "urban heat islands." All that concrete and asphalt soaks up the sun and keeps the city hot long after the sun goes down. This isn't just about being sweaty; it kills people during heatwaves. Smart cities are starting to plant "green roofs" and use lighter-colored pavement to bounce the heat back.
Infrastructure is the Skeleton
Everything in a city depends on the stuff you don't see. Sewers. Power grids. Fiber optic cables. When a city grows too fast, you get "squatter settlements" or "favelas." These are areas where people build their own homes on land they don't own. They don't have official addresses, so the city doesn't provide trash pickup or water.
Yet, these places are incredibly vibrant. They have their own internal economies and social structures. The challenge for 21st-century geographers is figuring out how to integrate these informal settlements into the formal city without just bulldozing them and making everyone homeless.
The Rank-Size Rule vs. Primate Cities
If you look at the US, the second-biggest city (LA) is roughly half the size of the biggest (NYC). The third-biggest (Chicago) is about a third. This is the "Rank-Size Rule." it usually means the country's economy is pretty well-distributed.
But look at France. Paris is way, way bigger than Lyon or Marseille. That’s a "Primate City." All the power, all the money, and all the culture are sucked into one spot. It’s great if you live in Paris, but if you’re in a rural village, you might feel like your part of the country is being ignored by the government.
Important Concepts to Master for the Exam:
- Threshold and Range: How many people do you need to keep a business open (threshold)? How far are people willing to drive for it (range)? You’ll drive 20 miles for a kidney transplant, but maybe only 2 miles for a taco.
- Christaller’s Central Place Theory: This explains why small towns are close together and big cities are far apart. It uses hexagons because circles overlap or leave gaps.
- The Gravity Model: Big cities attract more people, more trade, and more ideas, just like big planets have more gravity. Distance matters, but size matters more.
Actionable Steps for Mastering Unit 6
To actually "get" this unit, you need to stop looking at your textbook and start looking at your own town.
- Open Google Maps in Satellite View: Look at your closest major city. Can you see where the old industrial areas were? Usually, they're near water or old train tracks.
- Identify the Sprawl: Look for the "edge cities." These are the massive clusters of office towers and malls that sit right at the intersection of two major highways, miles away from the "real" downtown.
- Trace the Food Deserts: Look at a map of grocery stores in a nearby metro area and overlay it with income data. The gaps are where the geography of inequality is most visible.
- Practice the Models with Real Cities: Try to fit the "Gallactic City" model to Atlanta or the "Concentric Zone" to a smaller, older Midwestern city. You'll quickly see where the models fail, and that’s where the real learning happens.
The study of urban patterns isn't just for passing a test in May. It’s about understanding why your rent is so high, why the bus is always late, and why some neighborhoods feel like they’re thriving while others are being left behind. Geography is the "why" of "where," and in Unit 6, that "where" is exactly where most of us spend our lives.