Types of Thematic Maps in ArcGIS Online
One of the primary features of ArcGIS Online is the ability to quickly create and publish thematic maps for the web.
A thematic map is a map designed to convey information about a single topic or theme, such as population density or geology. This is in contrast to a reference map, which is a more-general map designed to show where geographic features are in relation to each other.
This tutorial focuses on the specific types of symbology that can be used when making thematic maps in ArcGIS Online. Symbology is the set of conventions, rules, or encoding systems that define how geographic features are represented with symbols on a map. Elements of symbology include the size, colors, and shapes of the symbols used.
This tutorial focuses on thematic maps created from vector data, which models (represents) the world as points, lines, or polygons.
The individual vector objects on a map are called features.
Thematic maps are created from one or more specific attributes in a set of geospatial data. Geospatial data is what is where and attributes are the what part of geospatial data. Along with the vector type being mapped, the data class of the attribute will help determine the most appropriate type of visualization to use.
Attributes can be classified into specific classes, and the data class of the attribute being mapped, along with the data vector type, will guide the choice of symbology used to visualize that data.
A unit is a fixed quantity (as of length, time, or value) used as a standard of measurement. For quantitative attributes, knowing the unit used for the values of that attribute implies what the attribute's data class is, and, therefore what kinds of thematic maps are appropriate or inappropriate for effective communication.
For example, with median household income in the US, the unit is usually dollars. Although dollars by themselves are an amount, since this is a median, its data class is central tendency, which can be meaningfully visualized in ArcGIS Online as a choropleth or graduated symbol chart.
In contrast, for population, the unit is number of people, which is a count. Counts generally should be visualized as graduated symbols rather than choropleths to avoid the zone effect, where large, sparsely-populated areas seem visually larger than small, densely-populated areas.
Points are stored as data using a single pair of coordinates: latitude and longitude. Sometimes a third value, elevation, is also included.
Points are useful for representing objects like vehicles or smartphone locations that occupy little or no area. Even for things like buildings or parks that do cover significant geographical area, representing those areas as points makes calculation of travel routes easier, and makes symbology easier when maps cover large areas or when quantitative values are being mapped.
Geometric Point Location Only
When all features are of the same type, they are sometimes mapped using the same type of symbol. For example, below are locations of Chinese restaurants in Spokane, WA. They are mapped with simple geometric symbols, which are simple shapes like squares, circles, triangles, etc.
In ArcGIS Online, points are symbolized as Location (Single symbol) with an attribute of Show location only.
Point Location Only: Pictograph Points
To add expressiveness or character to a map, location-only symbols can be pictographs, which are symbols that look like the phenomenon being mapped, such as diagrams of barns to depict the locations of barns of historical interest (Slocum et al 2009).
Point Location Only: Heat Maps
When mapping the locations of large numbers of points, mapping each point as a single symbol can create an illegible map that also takes a long time to load:
One way of visualizing a large number of points is a heat map, where clusters of points are represented as areas with differing colors depending on how many points are around that area.
Point Location Only: Clustered Point Maps
A second way of visualizing large numbers of points is to use the interactive clustering feature. The app groups clusters of points together into bubbles whose sizes vary depending on how many points are in that cluster. As you zoom in to a clustered map, the clusters are broken into smaller clusters, until you finally can zoom in to individual points. Clusters can also be browsed to show the attributes for all points in a cluster.
Points with Nominal Data
Nominal attributes are names or descriptions, usually represented in storage as strings of characters. Nominal data is usually represented on a map with labels.
For example, below is a map of Chinese restaurants in Spokane, WA similar to the location only map above, but in this case the points are labeled with the name of the restaurant.
Points With Categorical Data
Point maps of categorical data usually represent different categories with different geometric shapes, pictographs, or colors.
Legends are essential with categorical maps to specify what the different symbols mean.
Points With Quantitative Data
Graduated bubble maps represent quantitative attributes by the size of the circle.
Polygons are represented with a collection of coordinate pairs that define the outside boundary of an area. Polygons are useful for representing things like property boundaries or city boundaries that define an area.
Polygon Location: Single Symbol
As with points, if all polygons are of one type, they can be colored the same to show location only. For example, these are metropolitan statistical areas (MSAs) of over one million people that the NY Times started with as a short list in their analysis of cities Amazon was considering in 2017 for its second national headquarters.
Polygon Location: Arbitrary Colors
Polygons can sometimes be colored to draw visual contrast between areas. The colors have no specific meaning and are arbitrary.
To create this type of map in ArcGIS Online, you can symbolize polygons as Types (Unique symbols) on a nominal attribute.
A choropleth is a thematic map in which areas are distinctly colored or shaded to represent classed values of a particular phenomenon.
One example of choropleths colored based on a categorical variable are red-state/blue-state maps commonly seen during election season.
Relative Quantitative Choropleths
Quantitative attributes on polygons are commonly visualized by coloring polygons different colors based on the attribute.
Note that these maps use shades of only one or two colors so it is clear visually what is low and what is high.
Also note that these are only appropriate for relative values like proportions or central tendency, as explained below.
Absolute Quantitative Polygon Maps
Absolute values like population counts or levels of precipitation should not be mapped as choropleths. We tend to percieve of larger areas on a map as having greater influence or importance. This can cause large, sparsely-populated areas to seem more significant than small, densely=populated states. This can be dramatically demonstrated with the red-state/blue-state maps commonly used in the media to represent the partisan political divide in the USA. This is an example of the zone effect, where using different borders for zones used in analysis can change the results of that analysis.
One way to deal with this issue is to create a bubble maps that varies the color or types of symbols at the centers of polygons rather than modify the symbology of the polygons themselves.
In ArcGIS Online, these are symbolized as Counts and Amounts (Size) on an absolute quantitative variable.
Lines are represented by sequences of coordinate pairs. Lines are useful for representing things like roads or paths that are long and narrow
Line Location Only
Qualitative Line Maps
Line maps of categorical data similarly can vary line color, thickness, or type to distinguish between different categories of features.
Quantiative Line Maps
Line thickness is commonly used to represent quantitative attributes associated with lines.
Thematic maps usually need a base map (sometimes written as "basemap") to provide a background of geographical context for the content you want to display in a map. With a base map, the points, lines, or polygons of a thematic map become visually associated with the real world.
ArcGIS Online provides a number of options for base maps. The choice of which base map to use should be guided by the ultimate purpose for the map as well as by aesthetics. If the thematic map will be used to guide navigation, the road network should probably be part of the base map. On the other hand, if a map is primarily focused on the data, use of a lightly-colored base map with few features may be more appropriate to avoid distractions and increase the contrast between the thematic features and the base map, making the map easier to read and interpret.
Things You Can't Do With ArcGIS Online
While ArcGIS Online is an extremely useful tool for creating web maps, there are some things you can't do with it.