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Urban hierarchy a term that relates the structure of towns within an area. It can typically be illustrated by dividing towns into four categories:
- 1st-order towns
- 2nd-order towns
- 3rd-order towns
- 4th-order towns
1st-order towns provide the bare minimum of essential services, such as bread and milk. The services that 1st-order towns provide require only a very low threshold population to survive, which make them suited to small communities. Services that require more customers to remain viable are not found in 1st-order towns.
3rd- and 4th-order towns are larger cities and communities. They are home to services that people are willing to travel longer distances to get to, as they are more important or rarer. The services in 3rd- and 4th-order towns require large threshold populations to survive, which is why they are only found in more developed areas.
It is clear, therefore, that there should be more 1st-order towns than 4th-order towns, as 1st-order towns only require a small number of people in their hinterland to remain viable.
German geographer Walter Christaller proposed the concept of the K-Ratio to describe the number of towns in one order in relation to the next. While Christaller's model is rarely found, it still provides a useful rule for the establishment of towns, called central place theory.
If a country had a K-Ratio of 3, for example, this would mean that there would be 3 times as many towns in the order beneath the current one. For example, if a country had the following number of towns in the relevant orders:
- 1st-order towns = 27
- 2nd-order towns = 9
- 3rd-order towns = 3
- 4th-order towns = 1
The country or area would be said to have a K-Ratio of 3, as a large, 4th-order town had three 3rd-order towns within its hinterland, nine 2nd-order towns and twenty-seven 1st-order towns.
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