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 general-reference map which provides a general overview of information, often representing multiple variables.

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

2012 Presidential Election Results by Party

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

  1. Start the Map
  2. Acquire the Data
  3. Subset the Data (Optional)
  4. Symbolize the Features
  5. Label the Features (Optional)
  6. Present the Map
  7. Save Your Project

Start the Map

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.
Starting a New Project

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.

Adding a Map to an Existing Project

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 maps with meaningful names to avoid getting them confused.
  5. Modify the new map as desired.
Duplicating a Map in an Existing Project

Acquire the Data

Feature Services

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 and 2016 US presidential election. The data is provided as the Minn 2016 Electoral States feature service from the University of Illinois ArcGIS Online organization.

This data was originally sourced from the Associated Press via Politico and represents election-night returns that do not exactly reflect official final vote totals.

Adding Data From a Feature Service

Shapefiles

The ESRI shapefile is a file format developed by ESRI in the late 1990s. While the format has many limitations and is obsolete, it is a well-established format that works with a wide variety of software, so it is still commonly used to share geospatial data.

The term shapefile is a misnomer since a shapefile is actually a collection of at least three (and usually more) separate interrelated files that store the locational data, the characteristics associated with those locations, and other information about the data. Some common files associated with a shapefile include (listed by the file extension):

For convenience, all these files are usually compressed into a single .zip archive file for distribution on websites and servers.

This example uses a shapefile of ward boundaries in Chicago from the city's open data portal.

  1. Download the shapefile .zip archive from the website.
  2. In Windows Explorer, Extract the contents of the .zip archive file (0:40).
  3. Add Data and add the shapefile to the map (0:53).
  4. Change the projection to an appropriate projection if the shapefile seems to have come in as unprojected lat/long. WGS 1984 Web Mercator is a safe choice unless you have a specific need for another projection (1:12).
  5. Right click on the new shapefile layer and select Data -> Export Features. This will start the Feature Class to Feature Class tool. This step is needed to copy the shapefile data into the project geodatabase so it will be saved when you package your project (1:20).
  6. For Output Feature Class provide a short but descriptive name with no punctuation or spaces.
  7. Run the to save the shapefile data. This should add the data as a new layer to your map.
  8. Delete the old shapefile layer (2:15).
Adding Data From a Shapefile

Subset the Data (Optional)

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.

Subset Based On A Single Field Value

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, this shows features from the Minn 2012-2016 Electoral Counties 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 value IL.
  4. Apply the query and click OK to finish.
Performing a definition query for a single value in a single field

Subset Based On a Ranges of Values

Queries can also be configured to subset ranges of values.

For example this example we show the counties in the Minn 2014-2018 ACS Counties layer that are in Illinois. Unlike the electoral county layer above, this layer does not have a state field for selection. However, it does have FIPS (Federal Information Processing System) codes for the counties.

County FIPS codes are five digits, with the first two digits indicating the state and the last three digits indicating the county. For illinois the FIPS state code is 17, so we can perform a definition query where the FIPS field is greater than or equal to 17000 and less than or equal to 17999.

Isolating a features with a range of values in a single field

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 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. Modify the symbology for Unique Values based on the categorical variable you want to map. For this example we use the
  2. 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.
  3. You might consider different base maps, or perhaps no base map at all.
Creating a Choropleth With a Categorical Variable

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. Right click the layer to modify the Symbology and select the variable you are going to map.
  2. Choose an appropriate classification method. The default of Natural Breaks (Jenks) is usually a safe choice.
  3. Choose a Color Scheme for the categories.
  4. Consider other base map options.
Creating a Single-Color Choropleth With a Quantitative Variable

Two-Color Quantitative Choropleths

There are situations where the purpose of the map is to show divergence above or below a central value. In such cases, the use of two separate colors for high and low values in a diverging color scheme is effective.

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 a balance.

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. Right click the layer to modify the Symbology and select the variable you are going to map.
  2. Choose an appropriate classification method. The default of Natural Breaks (Jenks) is usually a safe choice.
  3. Create a custom Color Scheme with two colors at the extremes and grey in the middle.
Creating a Two-Color Choropleth With a Quantitative Variable

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 size indicated by the variable is not the same as the sizes of the areas, we get an incorrect impression of where the larger and smaller values are located.

This example is a bubble map of the count of Republican voters in the 2012 election by state. Unlike the red-state / blue-state choropleth, this shows that the large, sparsely-populated states are actually a less significant source of Republican votes than more densely-populated states.

Creating a Graduated Symbol (Bubble) Map

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.

Creating a Dot-Density Map

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. Add Data the cartogram layer.
  2. Turn off the base map since a cartogram distorts geography.
  3. Join the two layers so the electoral data can be displayed in the cartogram polygons.
  4. Adjust the Symbology to use the display variable.
Creating a Cartogram

Label the Features

If the geographic areas being depicted should be fairly obvious to most readers, labeling the features can be a distraction. However, you have a limited number of features and you suspect some audience members may not be familiar with the names of the areas depicted by the features, you may want to add labels.

Automatic Labels

  1. Right click on the layer in the Contents pane and click to turn on Label.
  2. If you need to change the attribute used for the labels, right click on the layer in the Contents pane, select Labeling Properties..., delete the existing Expression, and double click the attribute you want in the Fields box.
  3. Adding a Halo around the labels can help make them more legible over the base map.
Creating Automatic Labels

Annotation Layer

One challenge with labels is dealing with situations where labels for features that are close together overlap. ArcGIS Pro tries to move the labels around so they all fit, but the algorithm sometimes doesn't know enough about the data to make appropriate choices. In those cases, you need to create an annotation layer.

  1. Turn on labels and adjust the properties for how you want the labels to look.
  2. Right click on the layer and select Convert Labels To Annotations.
  3. For the Output Database use the project database, which will probably be your only option.
  4. Run the tool.
  5. On the Edit tab, click Modify.
  6. In the modification options, select Annotation.
  7. Move the labels where you want them.
  8. Right-click to add leader lines if needed.
Movable Labels in an Annotation Layer

Present the Map

Layout For a Figure

In order to use the map as a figure in a document (such as a report), you need to create a map layout.

  1. On the Insert tab, select Print Layout. The size of the layout is dependent on how big you want it to be in your printed document. For this example we use 4" x 6".
  2. Add a map frame for the main mapping area.
  3. Size the map to 4x6 so it fills the figure and remove the frame line in case you do not want it in your document.
  4. Add a legend with a background and border so it stands out over the map.
  5. Add a Legend.
    1. Double click the variable name and give it a meaningful name that will look good in the legend.
    2. Remove the Layer Name from the legend so there is only one heading.
    3. Adjust the decimal points if needed.
  6. Create a dynamic text box outside of the figure to remove the the service credit for the base map.
  7. Add inset map frames for any outlying geographic regions. In the case of the US, Alaska and Hawaii are states that should be included on your maps of the country.
  8. Add a north arrow.
  9. Share -> Export to a PNG file. Portable Network Graphics is a file format that preserves quality at the expense of being larger than some other formats. Quality is important if you are printing.
  10. Insert the map in your Word document as a picture and center it.
  11. Give figure a meaningful caption. You can get source information by right-clicking on the layer and selecting View Metadata
Choropleth Print Layout For a Figure

Create a Print Layout For a Standalone Map

If you are going to use the map on its own, such as for a standalone handout or poster, you can add map elements to the layout to format the complete page.

  1. On the Insert tab, select Print Layout. For this example we use an 8.5" x 11" sheet of paper and choose Landscape orientation since the US is wider than it is tall.
  2. Add a map frame for the main mapping area.
  3. Add a legend and place it below the main mapping area.
    1. Double click the variable name and give it a meaningful name that will look good in the legend.
    2. Remove the Layer Name from the legend so there is only one heading.
    3. Adjust the decimal points if needed.
  4. Rename the layer as a meaningful title and remove the heading (variable name) so the text in the legend is relevant.
  5. Create a dynamic text box outside of the figure to remove the the service credit for the base map.
  6. Add inset map frames for any outlying geographic regions. In the case of the US, Alaska and Hawaii are states that should be included on your maps of the country.
  7. Add a textbox with a meaningful title.
  8. Add a north arrow.
  9. Add marginalia with the cartographer, date, and data source.
  10. Share -> Export to a PDF file. Portable Document Format preserves the layout exactly as you created it, regardless of what machine you view the file on.
Creating a Standalone Choropleth Print Layout

Save the Project

Save A Project Package

When you are done with a project, you should save it as a project package on ArcGIS Online:

  1. This package will save all of your maps, layouts, and local data together so that you can reopen the package later on any computer if you need to modify or recreate any maps.
  2. You can share the package in case you want someone else to be able to use the materials you created in the project.
  3. Saving this to ArcGIS online will protect you from losing your data if something happens to your normal work computer.
Saving a Project Package to ArcGIS Online

Saving Failure

Project packaging will commonly fail with cryptic errors related to history. A typical error is error 00246 "Geoprocessing history items with errors cannot be included in this package."

Project packages contain a list of tool operations performed in the past so you can audit or recreate your work if needed. If any of those operations failed, or if the data sources have been modified or deleted, the history items are invalid and ArcGIS Pro will refuse to save the project package.

You can fix this by going into Analysis -> History and deleting history items (especially ones with red failure marks beside them), then trying the save again.

Fixing the 00246 Error

Reopen a Project Package

Reopening a Project From a Project Package

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 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.

Irregular Polygons vs. Bubbles

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.

The Modifiable Areal Unit Problem

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.

Nonstandard Colors

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 underemphasize the contrasts between areas.

Differences in Categorization

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.

Differences in Categorization

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.

Counts vs. Rates

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.

Creating a Box and Whisker Plot