Choropleth maps apply colour coding to graphically convey information about a geographic area. The region of the thematic map is divided into smaller areas, typically political boundaries, such as states or countries. Each area is filled with a colour or pattern that denotes the value of some measurement, such as population density or unemployment rate. The colour coding allows the viewer to quickly pick out patterns and extremes on the map.
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Things you need
- Numerical data for the region to be mapped
- Mapping or drawing tools or software
Prepare and normalise the data as needed. To achieve a fair comparison of each area on the map, the data should reflect information without regard to the size of the area or the overall population it contains. For example, if you want to show how populated an area is, population density (number of people per square mile) may better convey that information than population alone.
Create or obtain a blank base map. This map should show the region with the areas outlined. For example, if the data are for townships in a county, the base map should cover the county with the townships outlined. If the data are for counties throughout the United States, a base map with the outlined counties with a thicker or coloured outline around each state will help the audience locate specific areas on the map. Add minimal labels and additional information as needed to help the viewer interpret the map without making the map too cluttered. For example, label each area or show the locations of major points, such as cities.
Set a number of divisions for the data appropriate to the data, with equal ranges for each division. Depending on the data, you may want to choose an upper cut-off for which you will lump everything above that value into one group. For example, if you are working with per capita income, an appropriate scheme may be to divide the data into £13,000 boundaries and lump all values in excess of £65,000 into a single colour group.
Choose a colour for each division of the data. Typically, choropleth maps are essentially a comparison of "more versus less" or "higher versus lower." Graduated shades of the same colour (for example, maroon, red and pink), or between two colours (dark green, kelly green, lime green and yellow) can illustrate this type of comparison well. The deepest, darkest colour usually represents the highest values; the lightest colour represents the lowest values. Each colour should be distinct and easy to pick out on the map.
Colour each area on the map to match the colour assigned to the value range matching the data value for that area. Using the per capita income example, an area with a per capita income of £24,370 will be coloured using the colour assigned to the "$20,000 to £25,999" range. An area with a per capita income of £82,930 will be coloured using the colour assigned to the "over £65,000" range.
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