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.

2012 Presidential Election Results by Party

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 and 2016 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 the Associated Press via Politico and represents election-night returns that do not exactly reflect official final vote totals.

  1. From your ArcGIS Online Home page, click Map to create a new map.
  2. Add and Search For Layers. Where you search depends on where the layer is. For this example we use the Minn 2016 Electoral Counties layer from the University of Illinois organization.
Adding a layer 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 While the format has many limitations and is obsolete, it is a well-established format that works with a wide variety of software, so it is still commonly used to share geospatial data.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 to your hard drive.
  2. From your ArcGIS Online Home page, click Map to create a new map.
  3. Add and Add Layer From File with the downloaded shapefile.
Adding data from a zipped shapefile

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 definition query to select a subset of points based on criteria you specify.

Filter 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 how to filter features from the Minn 2016 Electoral Counties to show only counties in Illinois:

  1. Click the Filter icon.
  2. Choose the attribute for filtering. In this example we use the ST two-letter state abbreviation.
Adding a filter 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. Change Style to use the categorical variable. In this example we use the WIN2012 column for the winner of the 2012 US presidential election.
  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. Always click OK and Done. If you fail to do this, your changes will lost when you save your map.
  4. You might consider different base maps.
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. Change Style and select the variable you are going to map.
  2. If desired, change the Counts and Amounts (Color) Options to a different color.
  3. Always click OK and Done. If you fail to do this, your changes will lost when you save your map.
Creating a single-hue choropleth with a quantitative variable

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. Change Style and select the variable you are going to map. For this example we use the PCGOP2012 variable for the percentage of the vote that went to the Republican candidate.
  2. If desired, change the Counts and Amounts (Color) Options to a different color.
  3. Click the Classify Data box.
  4. Using the default Natural Breaks option is usually a safe choice unless you have reason to use another classification algorithm.
  5. 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.
  6. Always click OK and Done. If you fail to do this, your changes will lost when you save your map.
Creating a single-hue classified choropleth with 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. Change Style and select the variable you are going to map. For this example we use the PCGOP2012 variable for the percentage of the vote that went to the Republican candidate.
  2. Change the Counts and Amounts (Color) Options.
  3. Change the Theme to High to Low and select an appropriate color ramp.
  4. Always click OK and Done. If you fail to do this, your changes will lost when you save your map.
Creating a diverging color-scheme 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

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. Click on the ellipsis (...) beside the layer and select Create Labels.
  2. If needed, change the variable used for the labels.
  3. 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.
Creating labels

Save and Share Your Map

Save the map under a meaningful name and Share the map to get a link you can share with others or submit for an assignment.

Saving and sharing a map

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.

Reopening a map