Population Calculation Method The present population may be obtained from recent census with reasonable alterations. Future prediction is based on the knowledge of the city and its environments, commerce and industries and their expansion, development of surrounding countries, etc. Helpful in predictions will be the study of population trends of similar cities and consultations with local people and officials. The following seven methods are generally used for predicting population:

(1) Uniform Growth Rate Method : In this method, a constant increment of growth is added periodically. For example, if the population increased from 90,000 to 100,000 in a period of five years, it would increase by an increment of 10,00 in the next five year period. This method is also known as the Arithmetical Progression Method.

(2) Uniform Percentage Growth Rate Method : In this method, a constant percentage of growth is assumed for equal periods of time. For example, if the, population increased from 100,000 to 110,000 during the past decade, it would increase 10 per cent to 121,000 during the next decade.

(3) Decreasing Growth Rate Method : This method is similar to the uniform percentage growth but with an arbitrary assumption of a decreasing rather than a constant rate of increase.

(4) Graphical Extension Methods : In this method, the population time curve is extended into future date by eye-estimation. This method is also known as the Curvilinear method.

(5) Graphical Comparison Method: This method involves the extension of the population-time curve of the city under study into the future based on a comparison with population-time curves of similar cities. In the this method, the population-time curves are plotted as indicated in Fig. 2.3 with the curves for all cities passing through the same point, represented by the present population of the city for which the predication is to be made. Projections based on studies of migrations, and on other predicted factors may be used to modify the results of the graphical comparison method. This method gives promise of a reliable prediction as it is based on a logical study of the past and future conditions. This method is also known as the Modified Curvilinear Method.

In Fig. given below the population-time curve for the city A, is plotted upto the year 1960 in which its population was 40,000. City B reached 40,000 in the year 1930, so its curve is plotted from the year 1930 onwards. Similarly, curves are drawn for cities C, D, and E from the year they reached A's population of 1960 ; i.e. 40,000. The curve of the city A can now be continued allowing it to be influenced by the rates or growth of cities B, C, D and E.

(6) Empirical Method : The following empirical formula Was suggested by Hardenberg :

where Pf = future population

Pp=present population r = probable rate of yearly or per decade increase n = number of years to be considered. When the population data of the past decades are available the average

value of r can be computed from the following expression

where P1 and P2 are to population data at two dates of n number of years.

This method should be used carefully as it may give erroneously high results when applied to young and rapidly advancing cities hav-ing expansion of short duration only. This method is also known as the Geometrical Progression method.