Thematic Mapping - Study Notes

Lesson summary

Thematic Mapping is Unit 11 of Class 12 Geography (Practical Geography). It defines thematic maps, lists their purposes and types, and teaches how to construct and interpret a dot map and a choropleth map with worked examples.

1. Thematic maps

• A thematic map emphasizes a particular theme or special topic, such as the average distribution of population of an area.
• It differs from a general reference map, which shows general features like rivers, cities, political subdivisions and highways.
• It uses a variety of cartographic symbols to depict the spatial pattern of a particular geographic quality or quantity.
• It emphasizes spatial variation of one or a small number of geographic distributions: physical phenomena such as climate, or human characteristics such as population density and health issues.
• It displays density, distribution, relative magnitudes, gradients, spatial relationships and movements.
• In 1854, John Snow, a doctor from London, created the first thematic map used for problem analysis when he mapped cholera's spread through the city.

Purposes of thematic maps

1. They provide specific information about particular locations.
2. They provide general information about spatial patterns.
3. They can be used to compare patterns on two or more maps.

Five types of thematic map

They are useful to decision makers, analysts, storytellers and others who want insights from their data. This unit studies the dot map and the choropleth map.

2. Dot method

• A convenient method of representing absolute numbers. Each dot has a value; the number of dots in an area follows the distribution pattern.
• It gives a good visual impression, a realistic distribution pattern and helps in understanding correlations.
• Uses: distribution of population, cattle, area under crops and output of commodities when data is given in absolute figures, administrative unit-wise.
• Size of dots: round, uniform and small; about 1 to 1.5 mm in diameter. The dot value is fixed carefully, keeping in view the highest and lowest values.
• Placing of dots: must conform to geographic conditions. Sandy, stony and forested areas are unimportant for crops and settlements. Marshy areas, lakes and river beds are left without dots. Unproductive lands are called negative lands; shade them lightly with a lead pencil first and erase after placing the dots.
• Requirements: an outline map with boundaries of administrative divisions, kept as small as possible for accuracy; topographical sheets, irrigation, relief, soil and rainfall distribution maps also help.

Steps to construct a dot map

1. Obtain a base map of the area or administrative regions.
2. Study the data and decide a dot value (a rounded number).
3. Decide an appropriate dot shape and size; dots that are too small give an overly sparse pattern that is not precise.
4. Place the correct number of dots within each administrative boundary.

Example 11.1: population, 2019 (one dot = 1,000,000 persons)

3. Choropleth method

• Uses shades or tints to show the intensity or distribution of a particular element.
• Administrative units form the basis of spatial distribution of data, so shading conforms to administrative units.
• Light shading shows low density; dark shading shows high density.
• Uses: densities per unit area within political divisions, such as population per square kilometre or yield per hectare; density of population, literacy, growth rates and sex ratio; percentages, for example the percentage of area under wheat to the total cropped area.
• Requirements: a map showing the different administrative units, and statistical data for those units.

How to interpret a choropleth

1. Identify the feature being mapped.
2. Verify the value of each shade by reading the legend.
3. Identify the scale of the administrative regions.
4. Using the key, find areas with the same shading.
5. Describe the density within and between areas.

Steps to draw

1. Arrange the data in ascending or descending order.
2. Group it into 5 categories: very high, high, medium, low and very low.
3. Interval = Range/5, where Range = maximum value - minimum value.
4. Mark shades or patterns in increasing or decreasing order.

Example 11.2: literacy rate of Tamil Nadu, 2011

Highest: Kanyakumari 91.75%; lowest: Dharmapuri 68.54%; range = 23.21.