Theory question 202
Statistical Maps and DiagramsUse TABLE II below and the attached map A for Nigeria to answer the questions that follow.
TABLE II: Volume of Rail Passenger Traffic Generated by Five Nigerian cities, 1973
Ibadan: 5,000,000
Enugu: 3,000,000
Maiduguri: 2,500,000
Port Harcourt: 1,500,000
Minna: 1,000,000
(a) Represent the statistics with proportional circles. (b) Outline two advantages of representing data with proportional symbols.
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(a) Radius proportional to the square root of each value: √5,000,000 ≈ 2,236; √3,000,000 ≈ 1,732; √2,500,000 ≈ 1,581; √1,500,000 ≈ 1,225; √1,000,000 = 1,000. Choose a scale, e.g. 1 cm = 1,000 units of square root: radii Ibadan 2.2 cm, Enugu 1.7 cm, Maiduguri 1.6 cm, Port Harcourt 1.2 cm, Minna 1.0 cm; draw each circle centred on its city on the map of Nigeria, with title, key and scale.
(b) Gives a clear visual impression and easy comparison of quantities; shows the location (distribution) of the data on a map at the same time; a wide range of values can be shown in a small space; attractive and easy to understand.
Use square roots of the values so that circle areas, not radii, match the data.