Thematic Maps (GIS and Spatial Analysis) Part 1

A thematic map is a visual representation of characteristics of a given geographic location. The characteristics illustrated within the map may consist of a wide variety of properties of interest to the researcher. These include, but are not limited to, quantitative properties of the geographic region, such as population and demographic information, as well as qualitative properties, such as descriptive information about specific types of crime occurring in the region. Data for use in a thematic map may be from an existing source, such as census or police data, it may be original data collected by the researcher him or herself, or some combination of both.

Thematic maps may be used to provide such information as the difference in the number of homicides between one county and another through a single glance at a map. They can show the audience about the differences in income, age, and ethnicity of the population in a particular city. Information provided on thematic maps can be at the level of global region, state, county, city, or even as small as a census block group.

Researchers can use their own existing data sets within the GIS software to create thematic maps. The software allows you to present visual representations of statistical analyses of data in the form of a thematic map. For example, a researcher may instruct the GIS application to calculate the number of robberies per hundred residents for each census block group in a particular city. Once the calculation is performed, the GIS application provides different options for presentation of the results in a thematic map. The differing rates may be presented using different colors to allow the audience to easily distinguish between the rates of robberies in different block groups with a single glance.


The accuracy of the map is dependent upon the accuracy of the data, as with geocoding. Thus, the major work on the part of the researcher lies in data preparation as with any type of analysis. The researcher who creates a thematic map depicting the differences in the number of robberies per one hundred residents between census block groups must be sure that the data set used to provide information about robberies is accurate to begin with. If the data incorrectly reports the number of robberies, the resulting thematic map will subsequently report the incorrect number of robberies as well.

An additional consideration with GIS is the accuracy of the underlying reference map data. This is the data used by the application (as specified by the researcher) to represent the applicable geographic location. Typically, the reference data consists of maps of countries, states, cities, or any other geographic region. ArcGIS comes with some geographic map data that can be used as reference data. However, there are many third-party sources for data of this kind (including ESRI, the manufacturer of ArcGIS) that extends far beyond that of the data provided in the sample data sets with ArcGIS. Data of this nature may be divided along a variety of borders, including but not limited to: country borders, census boundaries, state lines, county lines, political boundaries, and so on. It is essential for the researcher to be aware of the nature of the boundaries of the reference data as this will have an effect on the analyses and subsequent results as presented in the thematic map.

Example: Creating a Thematic Map from Sample Data

Figure 1.60 shows a simple thematic map created from the sample data provided with ArcGIS. The map illustrates the difference in population between all 50 states in the United States according to the data for Census 2000. This map was created using a single field from a single layer of data provided with the sample data installed with ArcGIS. This is the most basic example of a thematic map. We will now go through the steps used to create this map, which you can follow along with if you have ArcGIS installed. Once we go through the steps to create the most basic type of thematic map, we will use the skills learned to build more complex thematic maps.

Figure 1.60 shows a simple thematic map created from the sample data provided with ArcGIS. The map illustrates the difference in population between all 50 states in the United States according to the data for Census 2000. This map was created using a single field from a single layer of data provided with the sample data installed with ArcGIS. This is the most basic example of a thematic map. We will now go through the steps used to create this map, which you can follow along with if you have ArcGIS installed. Once we go through the steps to create the most basic type of thematic map, we will use the skills learned to build more complex thematic maps.

Figure 1.60 Sample thematic map of the United States

Sample thematic map of the United States

Step 1 Use the Start menu on your computer to launch ArcMap.

Step 2 On the Welcome screen check the radio button for "A new empty map" and click OK.

Step 3 Use the Add Data button (dark cross on a light background) to browse to the location on your computer where the sample data is stored. By default this data will be stored at: \ArcGIS\Bin\TemplateData.

Step 4 Select the file called "states.shp" from the USA folder and click Add to add the file to ArcMap as shown in Figure 1.61.

Figure 1.61 Selecting a shape file

Selecting a shape file

A map of the United States should appear in your ArcMap dataframe as shown in Figure 1.62.

FIGURE 1.62 A basic map

A basic map

The sample data set "states.shp" is associated with data from the United States Census Bureau. To view the attributes associated with this file right-click the mouse on the layer name in the table of contents and select Open Attribute Table. Go ahead and use the scroll bar to scroll across the table and view the available information. For the purposes of this basic example we are going to use the field named "POP2000" to create a thematic map displaying the differences in population between the states according to the U.S. Census data 2000. Now, we will create the thematic map. Use the X in the corner of the attribute table to close the table.

Step 5 Double-click on the states layer name to open up the Layer Properties dialog box.

Step 6 Select the Symbology tab in the Layer Properties dialog. Within the Symbology tab, select Quantities from the show panel as in Figure 1.63.

FIGURE 1.63 Symbology tab options

Symbology tab options

You will see that by default the Graduated colors option is selected. However, there are the additional options of Graduated symbols, Proportional symbols, and Dot density that can be selected instead if these are more appropriate for your data. We will see examples of the graduated symbols and proportional symbols options later in this section. We will also explore some of the other available options for the Symbology tab in subsequent examples. Since we are currently interested in creating a simple thematic map showing the population differences between states according to the U.S. Census 2000, we need to specify which field from the attribute table ArcGIS should use to base the symbology upon.

Step 7 Use the pulldown menu next to Value in the Fields box to select the field "POP2000."

Figure 1.64 Selecting the variable to display

Selecting the variable to display

Step 8 Click Apply. The Symbol, Range, and Label fields will automatically populate according to the default settings for ArcGIS, as shown in Figure 1.65.

Figure 1.65 Details of the Symbology tab

Details of the Symbology tab

Step 9 Click OK.

The map will appear in the ArcMap dataframe, as shown in Figure 1.66.

FIGURE 1.66 A thematic map of state population, 2000

A thematic map of state population, 2000

If you compare these results to the initial map presented at the beginning of this section (Figure 1.60) you will notice that the two are different. The map you just created does not contain the title and the legend that appear on the initial map. These are features that are added to the map before presentation in order to make them more clear to the audience. These features cannot be added in the data view of ArcGIS. Instead, we must use the layout view in order to specify how we want ArcGIS to layout our map presentation.

Use the Layout View button on the bottom of the ArcMap dataframe to switch over to the layout view of your map (Figure 1.67).

FIGURE 1.67 The layout view

The layout view

The layout view will display on your screen. The layout view will show ruler guides along the left and top margins to provide a frame of reference for printing purposes. You can use the left mouse button to reset margins as needed. Now, we can add our title and legend to provide a more clear presentation of the data contained in our map.

Step 10 Use the Insert menu in the layout view and select Legend as shown in Figure 1.68.

FIGURE 1.68 Adding a legend to the map

Adding a legend to the map

Note that the Insert menu provides the option of inserting various components that may be useful in creating maps for presentation. These include north arrows, text, scale bars, and even pictures. Explore these options on your own to gain insight into the creative options available for map presentation.

The Legend Wizard will appear on your screen. Since we are only working with a single layer of data for this example it is not necessary to select which layers to include in the legend. In this case only the states layer appears in the Map Layers dialog and it is included in the Legend Items by default. If there were multiple data layers contained in our map we would need to use the Legend Wizard to specify which layers to include in the legend. Go ahead and use the Preview button to see a preview of the default legend ArcGIS creates for this map. It may be necessary to adjust the position of the Legend editor window in order to view the preview on the map layout underneath as shown in Figure 1.69.

FIGURE 1.69 The Legend Wizard

The Legend Wizard

Select Finish to add the legend to your map. Once the legend appears in the map you can use the left mouse button to move it around on the map until you find the desired position. You can also use the mouse to resize the legend. By double-clicking on the legend you will have access to the Legend Properties dialog (Figure 1.70). The Legend Properties dialog can be used to set specific properties of your legend. Take a moment to explore each of the four tabs to get a sense of the available options for the properties of the legend.

FIGURE 1.70 Legend Properties

Legend Properties

These properties may be used to customize your legend as well as make the items in the legend more clear to the specific audience to whom you are presenting your map. As with any method of data presentation, there may be different relevant aspects of the data you wish to highlight depending on the audience. Using GIS to present your data often means that a simple change in the legend presentation is all you need to tailor your data according to your audience.

The default legend style used to create the legend for our map includes the layer name. In this case, the layer name "states" seems unnecessary to include as part of the legend. We can specify that we are viewing the United States in our title instead. To remove the layer name from this legend, choose the Items tab in the Legend Properties dialog. Then, choose the Style button to see the available options for the legend style. Choose several different styles to see how they differ in appearance and information presented. After you have explored the various options, choose the style called "Horizontal with Heading and Labels" as shown in Figure 1.71.

FIGURE 1.71 Legend Item Selector

Legend Item Selector

This option eliminates the layer name while still presenting the name of the field used to be clear about what the categories in the legend represent. Click OK in the Legend Item Selector to apply the selected style to the legend in the map layout. Click OK in the Legend Properties to dismiss the dialog and view your map with the updated legend. The Legend Properties dialog provides a simple, straightforward method of adjusting the legend as needed.

Now, we are ready to add a title to our map to add to the clarity of the data presentation for the audience.

Step 11 Use the Insert menu in the Layout view and select Title instead of Legend.

Step 12 Type the following text in the title box on the map: "United States Populations by Census 2000."

Step 13 Use the Enter key on the keyboard once the title is typed in. The properties of the title can be edited in a similar manner to those of the legend. Double-clicking on the title text will launch the Properties box for the title. Here, properties such as text style, size, and position can be altered as needed. Take a minute to explore the available options in the Properties dialog for the title. Click Cancel when you are finished reviewing these options.

Step 14 Use the mouse to drag the title into a center position on the map layout until it appears as shown in Figure 1.72.

FIGURE 1.72 A thematic map with legend added

A thematic map with legend added

Congratulations! You have successfully created a thematic map clearly depicting the United States population according to the U.S. Census for 2000. If you want to export this map with the title and legend to a program such as Microsoft PowerPoint, you can use the procedure we followed in Figure 1.59. To start this process, click on File, and then on export map from the layout window.

Now that you have completed the map you should save it to a safe location.

Step 15 Use the File > Save As option to save this map document (.mxd) to a location on disk for retrieval at a later time. For reference, name the map "U.S. Population by Census 2000."

The basic steps used to create this simple map of the U.S. population are the building blocks needed to create GIS illustrations of more complex data and analyses.

One of the strengths of GIS is that it allows for the comparison of geographic units over time on a variable or set of observations with relative ease and clarity. As an exercise, follow the same steps described in the previous example and construct the same map of the U.S. population, only this time use the "POP1990" column in the attribute table shown in Figure 1.64. This will allow you to compare the map you just made in this example with the map for 1990; patterns of change during the decade between 1990 and 2000 will be readily apparent, much more so than if you looked at two columns in the spreadsheet or attribute table.

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