Choropleth Method

Choropleth Method

The choropleth map uses shades or tints to show intensity or distribution of a particular element. It takes into account administrative units which form the basis of spatial distribution of data. That is why shading conforms to administrative units. The density patterns are highlighted by light shading (low density) and dark shading (high density).

Uses of choropleth map

Choropleth maps are drawn to represent densities per unit area within political divisions. Thus, these maps show population per square kilometre or yield per hectare.

The choropleth maps are also drawn to depict the data characteristics as they are related to the administrative units. These maps are used to represent the density of population, literacy, growth rates, sex ratio, etc.

These maps also show percentages, for example, percentage of area under wheat cultivation to the total cropped area.

How to interpret a choropleth

1. Identify the geographic feature or phenomena being mapped.

2. Verify the value of each shade used on the map. This can be done by reading the map’s legend.

3. Identify the scale of the administrative regions shown on the map.

4. Using the key as a guide, identify the areas of the map that share the same colour shading and the same quantity volume of the feature being mapped.

5. Describe the density or concentration of the feature within and between different areas of the map.

Requirement for drawing Choropleth Map

a. A map of the area depicting different administrative units.

b. Appropriate statistical data according to administrative units.

Steps to be followed

a. Arrange the data in ascending or descending order.

b. Group the data into 5 categories to represent very high, high, medium, low and very low concentrations.

c. The interval between the categories may be identified on the following formulae i.e., Range/5 (Range = maximum value – minimum value).

d. Patterns, shades or colour to be used to depict the chosen categories should be marked in an increasing or decreasing order.

Example

Construct a Choropleth map to represent the literacy rates in Tamil Nadu as given in Table 11.2.

Table 11.2 Original Data

Literacy rate of Tamilnadu – 2011

Table 11.3

Table arranged in descending order

Construction

a. Arrange the data in descending order as shown above.

b. Identify the range within the data. In the present case, the districts with record of the highest and lowest literacy rates are Kanyakumari (91.75%) and Dharmapuri (68.54%) respectively. Hence, the range would be 91.75 – 68.54 =23.21.

c. Divide the range in to 5 to get categories from very low to very high.

d. Determine the number of the categories along with the range of each category. 

We will finally get following categories

e. Assign shades/pattern to each category ranging from lower to higher values.

f. Prepare the map as shown in Figure.