Urban Primacy is when one city is dominantly large in a group of cities. Usually when a largest city is over twice as large as the next city, this would constitute urban primacy. Other measures are more rigorous whereby a city is only primate when it is at least 3 times larger than the next two cities combined. Primacy is almost always calculated using this kind of population-size ratio. Ideally metropolitan level populations are used so as to avoid dividing the largest city by an adjacent centre.
The idea of primacy was first introduced by Mark Jefferson in 1939. His proposition was that nationalism crystallizes in primate cities which are super eminent in both size and national influence. He assessed the degree of primacy by computing the ratio of the size of the second and third ranking cities to that of the largest one. He found that in the forty-six countries of the world the largest cities were two or three times as large as the next largest city. The ratio of the population of the three largest cities approximated the sequence 100:30:20 ( i.e. the third largest is one-fifth the six of the largest). According to him there are various reasons for a city to exceed its neighbors in size, but once it did so the process became cumulative giving it an impetus to grow and draw away from all other cities in character as well as size. The particular ratio sequence has been later ignored, though the concept of the primate city and primacy is widely used.
A primate city is the largest city in its country or region, disproportionately larger than any others in the urban hierarchy. A primate city distribution is a rank-size distribution that has one very large city with many much smaller cities and towns, and no intermediate-sized urban centers: a King effect, visible as an outlier on an otherwise linear graph, when the rest of the data fit a power law or stretched exponential function. The law of the primate city was first proposed by the geographer Mark Jefferson in 1939. He defines a primate city as being “at least twice as large as the next largest city and more than twice as significant.” Aside from size and economic influence, a primate city will usually have precedence in all other aspects of its country’s society, such as being a center of politics, media, culture and education and receive most internal migration.
Primacy Index
PI= C1(C1+C2+C3)*100
C1– Population of first-rank city
C2– Population of the second rank city
C3–Population of third rank city
if PI is greater than or equal to 50, primacy is there.
The Rank-Size Rule
Zipf’s has probably the best presentation of the empirical findings on rank and size of the cities. The rank size rule states that for a group of cities, usually those exceeding some size in a particular country, the relationship between size and rank of cities is given by:
Pr = P1/r
Where Pr = population of the largest city ranked r
P = population of the largest city
r = rank of city r
Rank Size Rule is a simple model which states that population size of a given city tends to be equal to the population of the largest city divided by the rank of the given city.
Rank-size rule analysis and ancient cities
The Rank Size Rule applies to the modern cities but when we try to apply the same law to the older cities we face difficulty in correlating the theory. The geographers try to correlate the data with the population datum which we don’t have in case of ancient cities and ancient city were characterized by city walls or fortifications but the settlement was never within the city walls, it always exceeded. The population of the city will depend on the city area. A larger city will have larger population.
Link(s) and Source(s):
Rashid's Blog: Portal for Inquisitive Learners 