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Creating Thematic Area Maps With ArcGIS Online

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.

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

1. Acquire the Data
2. Filter the Data (Optional)
3. Symbolize the Features
4. Label the Features (Optional)
5. Save and Share the Map
6. Reopen a Map

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 host for feature services in the ESRI ecosystem is ArcGIS Online, which is tightly integrated with their product line. 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 provided as the Minn 2016 Electoral Counties feature service from the University of Illinois ArcGIS Online organization. This data was originally sourced from state secretary of state offices.

1. From your ArcGIS Online Home page, click Map to create a new map.
2. If you are placed in the new map viewer (with panels on both sides), click the link at the top to Open in Map Viewer Classic.
3. Select Add and Search for Layers.
4. Where you search depends on where the layer is. For this example we search My Organization (the U of I organization) for Minn 2020 Electoral Counties.

Shapefiles

There are a wide variety of different types of files that can be used to store geospatial data, although only a handful are commonly used.

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 While the format has many limitations and is obsolete, it is a well-established format that works with a wide variety of software, so it is still commonly used to share geospatial data.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):

• .shp: Contains the feature geometry (points, lines, polygons)
• .shx: An index file that indicates where specific features are in the .shp file
• .dbf: A dBase IV database file of attributes associated with each of the shapes in the .shp file
• .prj: The coordinate system and projection used by the feature geometry (optional)
• .cpg: The character encoding used by the attributes (optional)
• .qpj: The coordinate system and projection in a format used by QGIS (optional)

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 to your local computer.
2. From your ArcGIS Online Home page, click Map to create a new map.
3. Select Add and Add Layer from File to add the shapefile from your local computer.

Filter 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 filter to select a subset of points based on criteria you specify.

For example, this shows how to filter features from the Minn 2020 Electoral Counties layer to show only counties in Illinois:

1. Click the Filter icon under the layer that you want to filter.
2. Add expression.
3. Select the condition for filtering. In this example we use the ST two-letter state abbreviation under a condition that it is the abbreviation for Illinois (IL).

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. In the Contents pane, under the layer select the Change Style icon.
2. Choose an attribute to show by selecting the field in the data that you will use to drive styling. In this example we use the Winner 2012 column for the winner of the 2012 US presidential election.
3. Select the Types (Unique Symbols) drawing style.
4. If needed, select the style Options and 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.
5. Always click OK and Done. If you fail to do this, your changes will lost when you save your map.
6. You might also consider different base maps to make the features stand out more clearly (figure-ground relationship).

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 Republican 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. In the Contents pane, under the layer select the Change Style icon.
2. Choose an attribute to show by selecting the field in the data that you will use to drive styling. In this case we use Percent GOP 2012 with the percentage of Republican votes in the 2012 presidential election.
3. For a choropleth, use the Counts and Amounts (Color) drawing style.
4. If desired, change the style Options to select different colors.
5. You might also consider different base maps to make the features stand out more clearly (figure-ground relationship).

Classified Quantitative Choropleths

By default, ArcGIS Online symbolizes quantitative variables as a continuous range of colors. While this is useful for showing patterns and may be adequate for some variables, it also makes it difficult to tell what the value is for any particular area. In cases where the different values are not evenly distributed (spread out) over the full range of possible values, this can exaggerate or hide meaningful clumps of values.

Because maps are subjective interpretations of data, there is no one single, objective solution to this issue. However, the use of a limited number of classes (ranges) of values represented by distinct colors can help.

1. In the Contents pane, under the layer select the Change Style icon.
2. Choose an attribute to show by selecting the field in the data that you will use to drive styling. In this case we use Percent GOP 2012 with the percentage of Republican votes in the 2012 presidential election.
3. For a choropleth, use the Counts and Amounts (Color) drawing style.
4. Change the style Options to select different colors.
5. Click the Classify Data box.
6. Using the default Natural Breaks option is usually a safe choice unless you have reason to use another classification algorithm.
7. Change the number of classes as desired. Five to seven categories usually gives a meaningful number of classes while not overwhelming the viewer with more information than they can efficiently understand.
8. You might also consider different base maps to make the features stand out more clearly (figure-ground relationship).

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. In the Contents pane, under the layer select the Change Style icon.
2. Choose an attribute to show by selecting the field in the data that you will use to drive styling. In this case we use Percent GOP 2012 with the percentage of Republican votes in the 2012 presidential election.
3. For a choropleth, use the Counts and Amounts (Color) drawing style.
4. Change the style Options.
5. For the Theme choose Above and Below.
6. Change the Symbol and select the desired color ramp.

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.

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.

1. In the Contents pane, click the ellipsis (...) beside the layer and select Create Labels.
2. Choose the Text field you want to label the areas.
3. Using a Halo will usually make the labels easier to read.
4. If the base map has labels that conflict, you might consider removing them. With the Light Gray Canvas base map, the World Light Gray Reference sub-layer is the labels on the base map.

Save and Share Your Map

Save the map under a meaningful name.

Share to change the sharing to an appropriate level of access. Everyone allows anyone to access the map.

Copy the link that you can share with others or submit for an assignment.

Reopen a Map

If you want to reopen a map in the future to make modifications, you can find it in the Content page of your ArcGIS Online account.