Explain the method of drawing isopleths on a map when point data are available. An imaging map is given here.

Method of Construction:

Isopleths are drawn on the map when point data are available. Fix the relative positions of maximum points on the map and put their values.

Let us suppose that we are given the location of several points along with their temperature in °C, on an imaginary map as shown in Fig. The minimum value is 4°C and the maximum value is 26°C. Suppose we have to draw isotherms at 5°C interval. The isotherm of minimum value will be of 5°C while that of maximum value will be of 25°C. Two points of 5°C temperature are already given of the map. We can find out the position of the third point having 5°C temperature by method of interpolation. For this purpose join point A of 10°C temperature with point B of 4°C temperature. Divide the line AB into 10 - 4 = 6 equal parts and locate the point having 5°C temperature by counting five divisions from A or one division from B. Join all the three points having 5°C temperature by a curved line. This line will be the isotherm of 5°C. Similarly, interpolate the points of 10°C temperature between C and D, and 15°C temperature between E and F and G and H and complete the isotherms of 10°C and 15°C respectively. Draw the rest of the isotherms in a similar manner and complete the map as shown in Fig.

Sometimes, the space between isopleths is shaded or coloured whose intensity increases with the increase in value. This brings in more clarity of the distribution. At times, tints of the same colour are also used.

Isopleths are used to show the following :

1. Climatic elements and relief features.

2. Density of population.

3. Distribution of crops.

Care should be taken to convert the data into ratios, percentages or index of concentration before actually plotting them on the map. For example, if we want to show the density of population in India, the density for each district will have to be calculated by dividing the population by its area. This will facilitate the drawing of isopleths. Similarly area under a particular crop is shown as percentage of the total cropped area.