Models are used to simplify reality and help us understand what is usually a whole range of complex processes. It is often useful to compare towns or cities to different models.
You need to remember that models are what could be happening and very often your job will be to show that you understand them, can explain them and can discuss whether or not they are good models. You do this last part by evaluating them and referring to case studies.
There are many rules that try and explain urban hierarchies.
You should have a general understanding of rank size rule, which introduces the concept of urban primacy and Christaller's central place theory.
Rank size rule: This tells us that in any country there will be one dominant settlement. After this, each settlement will decrease in size according to the rule.
This rule is that the "size of settlement 'A' will be equal to the population of the largest settlement divided by rank of 'A'."
Therefore, if the largest city has a population of 1,000,000 then the second largest city will have a population of 500,000 then 333,333 then 250,000 and so on.
There are two exceptions to the rule. We can have binary distribution when there are two very large dominant cities (Barcelona and Madrid) and primacy where one city is much larger than any other.
This is a key concept. A primate city is one that has more than twice the population of the next biggest city. An example is Lima (Peru) that is more than ten times larger than the next settlement. It can occur for a few reasons:
- One city will start to attract the majority of public and/or private investment. This could be due to natural advantage or political decisions. This in turn will stimulate further investment due to the multiplier effect and significant rural to urban migration. The investment in this city will be at the expense of other cities.
- A former colonial city is likely to maintain and possibly increase its urban primacy. It would have originally been the main source of colonial investment because colonial powers were not interested in investing in all cities, just those necessary for them to exploit the raw materials. Therefore, they probably invested heavily in a port. Today, this port will still be the focus of investment and migration.
- A country particularly dependent on export earnings will focus its development on one key port.
You will have noticed that the above three points seem typical of developing world cities and it is true to say that in most cases, as a country experiences economic development, so urban primacy is reduced or disappears totally.