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Creating Thematic Area Maps With ArcGIS Pro

A thematic map "is used to display the spatial pattern of a theme or attribute" (Slocum et al. 2009, 1). This is distinct from a reference map (like Google Maps) which provides a general overview of information, often representing multiple variables.

A commonly-seen thematic map in the USA is the red-state / blue-state choropleth map showing the predominance of political parties in presidential elections.

This tutorial covers the basic steps for creating thematic maps of areas in ArcGIS Pro.

Creating and Managing Maps

Start a New Project

To create a new map in a new project:

1. Log in to ArcGIS Pro.
2. When presented with options on the start screen, start your new project with a Map.
3. Give the project a meaningful title so you can keep track of what is in different projects.
4. Right click on the new map in the Contents pane and under Properties, give it a meaningful name (Thematic_Map).

Add a Map to an Existing Project

If you have an existing project you can add an additional map to the project by going to the Insert tab and selecting Map.

You may want to rename your maps so it is clear what map is what.

Duplicate an Existing Map

If you want to make a map similar to a map you already have in your project, or just want to make a revised version of an existing map without destroying the old one:

1. View the Catalog Pane.
2. Copy the existing map.
3. Paste to create the new map.
4. Rename the duplicated map with a meaningful name to avoid confusion.
5. Double click on the new map in the Catalog Pane to show the map in a new tab.

Delete an Unused Map

If you have prior versions of maps that you do not need, you should consider deleting them to avoid confusion if you or another operator needs to work with your project in the future.

Note that simply closing a map tab does not remove the map from a project.

1. View the Catalog Pane.
2. Right click on the unneeded map and Delete.

Acquire the Data

Feature Service

Data sources that wish to make their data available to the public often make it available as feature services. One major provider of feature services in the ESRI ecosystem is ArcGIS Online, which is tightly integrated into ArcGIS Pro. Some services in ArcGIS Online are provided by ESRI, while others are provides by organizations or individuals that use ArcGIS Online to disseminate their data privately or to the general public.

The examples in this tutorial use electoral data from the 2012 US presidential election.

• The data is available in the Minn 2020 Electoral Counties feature service from the University of Illinois ArcGIS Online organization.
• This data is also available as a GeoJSON file.
• The data was originally sourced from state secretaries of state offices, and metadata is available here.

Subset the Data

Subset a Single Value

There may be situations where you only need to display only some of the features in a layer. For those situations you can use a definition query to select a subset of points based on criteria you specify.

If you want to only work with features that have a specific value for a single field, you can perform a definition with a single clause.

For example, to show features for only counties in Illinois:

1. Right click on the layer you want to subset and click Properties and Definition Query.
2. Add a New definition query.
3. Add the conditions you want to define what features will be displayed. For this example, we choose the features where the ST state abbreviation field is equal to the desired value (IL).
4. Apply the query and click OK to finish.

Subset Based On Multiple Values

For this example, we use a definition query to subset Midwestern states adjacent to Illinois (IA, WI, IN).

Symbolize the Features

The choice of how to symbolize the data is based on the characteristics of the data as well as the particular story that you want to tell with your map.

The following subsections describe how to use a variety of different symbologies for different types of variables.

Categorical Choropleth

A choropleth is a type of thematic map where areas are colored based on a single variable that describes some characteristic of those areas. Choropleths can be used to visualize both categorical and quantitative variables.

The following video shows how to create a choropleth using a categorical variable.

1. Rename your map to something descriptive (Winner 2012).
2. Modify the symbology for Unique Values based on the categorical variable you want to map (Winner_2012).
3. Choose colors for the categories. In this case we use the standard highly-saturated red / blue palette common for maps of this type in the media since 2000.
4. Remove the all other values entry.
5. Under the Feature Layer ribbon, select Layer Blend and Multiply so the choropleth colors the base map and allows the base map symbols to be visible through the choropleth polygons.

Single-Color Quantitative Choropleths

Choropleths can also be used to visualize quantitative variables. When displaying a single variable, it is common to use a sequential color scheme with a range of lightness or saturation of a single hue that clearly conveys high versus low.

This example uses the percentage of the Democratic vote in the 2012 election. In contrast to the stark, divisively categorical red-state / blue-state maps, this type of map shows that there are Democratic voters in all 50 states.

While this map is not as effective for communicating election results as the red-state / blue-state map (where there is indeed only one winner), this map is more effective at communicating the complexities of the US electorate.

1. Rename your map to something descriptive (Percent Dem 2012).
2. Right click the layer to modify the Symbology for Graduated Colors and select the variable you are going to map (Percent_Dem_2012).
3. Choose an appropriate classification method. The default of Natural Breaks (Jenks) is usually a safe choice.
4. Choose a Color Scheme for the categories.
5. Remove the unnecessary decimal points from the legend entries.
6. Under the Feature Layer ribbon, select Layer Blend and Multiply so the choropleth colors the base map and allows the base map symbols to be visible through the choropleth polygons.

Two-Color Quantitative Choropleths

There are situations where the purpose of the map is to show divergence above or below a central value.

A diverging color scheme uses a range of colors between two opposite hues separated by a neutral color like white or gray. The use of the two different hues makes it easier to distinguish between areas with lower and higher values while still being able to see intermediate values between the extremes of the range.

An example of this is US election data, where most voters choose between two candidates from two opposing parties. Using the percentage of the Democratic vote by state, red for low values represents more people voting for Republican candidates, while blue for high values represents more people voting for Democratic candidates. The unsaturated grey in the middle indicates areas that are balanced between the two parties.

To make it easier compare and contrast groups of similar values, quantitative variables are often classified into ranges of values that are assigned to a limited number of different colors.

Like the single-color map, this map offers a nuanced view of the electoral landscape. However, the two-color map also points out balanced "swing" areas where efforts at political persuasion can be effective for winning elections.

1. Rename your map to something descriptive (Classified Dem 2012).
2. Right click the layer to modify the Symbology to Graduated Colors and select the variable you are going to map.
3. Choose an appropriate classification method. The default of Natural Breaks (Jenks) is usually a safe choice.
4. Create a custom Color Scheme with two colors at the extremes and grey in the middle.
5. Remove the unnecessary decimal points from the legend entries.
6. Under the Feature Layer ribbon, select Layer Blend and Multiply so the choropleth colors the base map and allows the base map symbols to be visible through the choropleth polygons.

Graduated Symbol Maps

One approach for mapping quantitative values for irregularly sized areas (like states) is to use a graduated symbol map rather than a choropleth. A common example of this is the "bubble" map that uses differently sized circles based on the variable being mapped. Although circles are most common, other types of icons can be used for aesthetic variety.

Graduated symbol maps are also more appropriate than choropleths when mapping counts rather than amounts (rates). Counts are variables that indicate size, such as the size of the population. With choropleth maps our eyes see the land area as the size, and when the sizes indicated by the variable are not the same as the sizes of the areas, we get an incorrect impression of where the larger and smaller values are located.

1. Rename your map to something descriptive (Bubble Dem 2012).
2. Right click the layer to modify the Symbology to Graduated Symbol and select the variable you are going to map.
3. Choose an appropriate classification method. The default of Natural Breaks (Jenks) is usually a safe choice.
4. Choose an appropriate color and bubble size scaling.
5. Remove the unnecessary decimal points from the legend entries, or add thousands separators as needed..
6. Under the Feature Layer ribbon, select Layer Blend and Multiply so the choropleth colors the base map and allows the base map symbols to be visible through the choropleth polygons.

Dot Density Maps

Another approach for mapping counts is the dot density map, where individual dots represent a certain portion of the overall count.

In this example, this allows us to map the counts of both Republican and Democratic votes simultaneously.

The disadvantage with a dot density map is that dots imply specific locations. Because the dots are distributed randomly across the area, this map does not accurately convey the exact spatial distribution of the voters. This can be remedied by using data for smaller areas (like counties), although data for smaller areas can sometimes be more difficult to acquire and less accurate for sparsely-populated areas where people are difficult to poll.

1. Rename the map with a meaningful name (Dot Density 2012 )
2. Change the symbology type to Dot Density.
3. Add fields (Votes_Dem_2012 and Votes_Rep_2012).
4. Change the dot colors as needed.
5. Adjust the dot value if one layer of dots overwhelms the other (5000).

Cartograms

Another solution to the irregular area problem is to create a map where the colored polygons are resized and reshaped based on population. This creates significant geographic distortion and is less of a map than a map-like graphic.

This example uses polygons for a continuous cartogram of US states sized by population in the Minn 2020 Cartogram State Continuous layer in ArcGIS Online. This and other cartograms are available here.

1. Rename your map to something descriptive (Cartogram Dem 2012).
2. Run the Feature Class to Feature Class tool to copy the cartogram polygons into a new feature class in the project geodatabase (Cartogram_States).
3. Turn off the base map since a cartogram distorts geography.
4. Run the Join Field tool to join the electoral data to the cartogram polygons.
5. Adjust the Symbology to use the display variable.

Present the Map

Once you have your map set up, you need to create a layout that arranges the mapped area with associated mapping elements that convey information needed to fully interpret the map.

Standalone Print Layout

If you want to print your map to paper or include it in a presentation in a manner where the map stands on its own with no additional content, you need a standalone layout that includes descriptive map elements.

Procedures for creating standalone layouts are described here.

Figure Layout

If you wish to include your map as a figure in a report or other kind of document that provides context for the map, a figure layout with a minimal set of map elements will be less cluttered and use document space more efficiently.

Procedures for creating figure layouts are described here.

Save the Project

When you are done with a project, you should save it as a project package on ArcGIS Online so you have a backup of your project, and so you can easily reopen the project on a different desktop machine if needed.

Procedures for saving a project package are described here.

Lying With Maps

Mark Monmonier's (1991) book How to Lie with Maps details a number of ways that cartographic choices are also choices about what story a map tells. Both the comparative ease of map making facilitated by software like ArcGIS Online and ArcGIS Pro, in tandem with the subtle complexity of standard cartographic techniques, makes it fairly easy to intentionally or unintentionally to tell stories with maps that may not be justified by the underlying data.

The Uneven Density Problem

Because we visually associate smaller and larger sizes with lower and higher values, creating choropleths of areas with widely differing levels of population density can create a mistaken interpretation of the variable being mapped.

The classic example of this is the red-state / blue-state map. Partisan choropleths can create a mistaken impression of Republican dominance since the land area of the US is dominated by sparsely-populated rural areas that lean Republican, while much of the population lives in smaller, densely-populated urban areas that lean Democratic but appear less dominant in a choropleth.

As described above, the uneven density problem be mitigated with bubble maps, dot-density maps, or cartograms.

The Modifiable Areal Unit Problem

The choice of different types of areas, such as counties vs. states, can alter the results of your analysis of the exact same phenomenon on the ground. For example, the impression given by a red-state / blue-state map is very different than one based on counties or congressional districts. Smaller areas expose fine-grained differences better than than larger areas, but make it harder to see broad patterns.

Unusual Colors

Nonstandard color choices can create a reversed impression of what the data actually represents. For example, creating a red-state / blue-state map where red is used for Democrats and blue is used for Republicans would be misleading in contemporary America.

Ill-Fitting Categorization Schemes

Likewise, the use of categorization schemes or thresholds that do not fit the distribution of the values in the data can over- or underemphasise the contrasts between areas.

The Classification Problem

In some cases, using categories rather than continuous color schemes can create clear distinctions when the situation is actually more nuanced. Again, the red-state / blue-state map implies that everyone in a red state is a Republican and vice versa, falling into the ecological fallacy. While such a map is reflective of the binary nature of the electoral college, if you are trying to actually map the political landscape, mapping continuous values emphasizes the purpleness of US society.

Counts vs. Rates

For data like health conditions, mapping counts as choropleths can make the situation in dense, populous areas seem more serious than it may actually be. Normalizing counts into rates, or using graduated symbol or dot-density maps can address this issue.

Is A Map The Appropriate Visualization?

You should ask if reducing an issue to Cartesian "where" is actually a meaningful representation of the situation. While proximity is still important, advances in communication and transportation mean that distantly separated groups and individuals can have closer social connections than people who are physically closer.

For example, a map of election results by county will show the rural/urban divide, with rural areas leaning toward the GOP and urban areas leaning toward the Democratic Party. However, this requires some understanding of the urban geography of the USA and is inexact. A box-and-whisker plot showing the % GOP vote based on level of urbanization (1 = highest, 6 = lowest) shows much more clearly the dominance of Democrats in urban counties (1) and the GOP in rural counties (6).

The following video shows how to make such a chart.