Creating Choropleth Maps in ArcGIS
This method example will make use of a polygons shapefile containing the boundaries and population data of the States in the USA called states.zip. Click [here] to download the zip archive. After saving the zip archive to a local working folder and unzip its content, start ArcMap.
Mapping Categorical Data
First, we will create a choropleth map which shades the states according to which sub-region they lie in (e.g., Pacific, New England, Mid Atlantic, etc.). Add the states shapefile as a layer by clicking File > Add Data and selecting the relevant file (i.e. states.shp). Double click on the states layer (or right-click on it and select "Properties"). This will bring up the Layer Properties dialog box.
Click on the "Symbology" tab. Then select Categories > Unique Values and select SUB_REGION from the Value field. In this case, since we are mapping categorical data, ArcMap has automatically selected a colour scheme which is appropriate for mapping this type of data, however if you would like to change the colour scheme you should do so now. Then click on "Add All Values" (bottom left) and then "OK". This produces a map where the different sub-regions are shaded in distinct colours. The legend on the left hand side shows the category (in this case sub-region) associated with each of these colours.
Mapping for Continuous Data
You could also create a choropleth map for continuous data. Let's try shading each state according to its population density (specifically, 1990 population per square mile, which is the variable POP90_SQMI).
Again, add the states shapefile as a layer by clicking File > Add Data and selecting the relevant file (i.e. states.shp). Double click on the states layer (or right-click on it and select "Properties") to bring up the Layer Properties dialog box. Click on the "Symbology" tab again, but this time, select Quantities > Graduated Colours and then select POP90_SQMI from the "Value" field. Select an appropriate colour scheme.
By default, ArcMap will split the continuous variable into five classes using the Natural Breaks classification scheme. This scheme works by selecting classes (based on break points) which best group similar values and maximize the differences between classes.
If you click on Classify on the top right hand side you will see a histogram of the attribute data values. The vertical blue lines indicate the currently selected class boundaries. You can adjust these boundaries interactively by dragging them with the cursor, or by typing in different break points into the "Break values" window.
You can change the classification scheme by selecting the relevant scheme from the Classification Method drop down list at the top right of the window. Experiment with some different classification schemes and check that you understand how they work. Commonly used schemes include the Equal Interval and the Quantile schemes. It is very important to recognise that choice of classification scheme can greatly affect the spatial patterns displayed in the map. Further information about these schemes is given in the ArcGIS desktop help.
Click on "OK" in the Classification window and then again on "OK" in the Layer Properties window and your map will be displayed.
The map below is a choropleth map of the states of the USA, showing population density (persons per square mile) in 1990, shaded in five classes according to the quantile classification scheme (i.e. quintiles).
ArcGIS® and ArcMap® are registered trademarks of Environmental Systems Research Institute, Inc. (ESRI). The software screen shots shown in this page are reprinted with permission from ESRI.