An election map is an example of a predominance map. It simply tells you which of two or more candidates have the highest total vote count.
For example, this map shows the predominant count of votes for President in North Dakota voting precincts from the 2008 election.
Areas with more Republican votes are in red
Areas with more Democrat votes are in blue
White represents an exact tie
Darker colors indicate a higher predominant vote for that party. Lighter colors indicate that party barely won the precinct. These measures represent the strength of the predominance over the other attributes.
Smart mapping lets anyone quickly discover patterns from attributes in their data and make meaningful maps from them. You can use smart mapping to analyze multiple columns of related data, in order to determine which has the highest, or predominant, value for each feature. Try a predominance map anytime you see multiple columns of data that share a common subject and unit of measurement.
For example, I have data regarding the crop output of US counties. Within my table, there are five attributes which track the amount of harvested acres by the type of crop:
I can use predominance to determine which crop type produced the most yield. It compares the values of my five attributes and immediately tells me which one is the largest value. The level of transparency tells me HOW predominant the attribute is compared to the other attributes. You can see this in the legend as the "Strength of predominance".
Predominance can also provide a sum of the five attributes in order to provide context about how many total acres were harvested from these five crops. This is represented by the size of the symbol, and can be seen in the map's legend as "Sum of categories".
To make a predominance map within the ArcGIS Online and ArcGIS Enterprise Map Viewer, simply select up to ten attributes within the smart mapping interface. To do this, go to Change Style and choose to "add attribute". Smart mapping will provide the "Predominant Category" and "Predominant Category & Size" options.
Let's quickly walk through an example together:
First, access smart mapping via the "Change Style" button within the layer options.
Then, begin to choose the fields you want to compare using predominance.
Continue to choose your attributes by using the "add attribute" button.
Once you have chosen your attributes, you'll see that smart mapping automatically suggests the two predominance styles:
- Predominant Category
- Predominant Category & Size
Selecting one of the predominance styles will produce a map which automatically shows you the predominant attribute.
This example shows the Predominant Category drawing style.
To learn more about the predominant attributes, go into Options.
Within Options, you can see a breakdown of how many features fell into each predominant category.
You also have the option to show "Other", which represents the features which had a tie between two or more of the attributes. This means that the attributes had the same value.
Within Options, you can also explore the transparency being used on the features.
By default, smart mapping will draw the features by the strength of their predominance over the next largest attribute.
You can use the sliders and percentages to adjust the amount of transparency being used.
When choosing a predominance style, you can also select "Predominant Category and Size".
Within the legend, you can see that the size of the symbol is directly related to the sum of the attributes.
When working with Predominant Category and Size, the Options button allows you to make changes to both the colors and sizes of your symbols.
If you want to compare more than ten attributes, you can do so using an Arcade Expression to calculate the predominant value.
Arcade is a simple expression language that allows you to create custom cartography, attributes, labeling, and more. The following blogs introduce Arcade and how to use it to create a predominance map similar to what is created by smart mapping:
Use Arcade Expressions to Map Your Ideas
Creating a Predominance Visualization with Arcade
Check out this map for an example of the settings used when created a custom predominance map. You'll notice that the pop-up also includes the predominant category. Use an Arcade Expression to create a clear pop-up describing the predominant value, which helps your audience better understand the map. Including a chart in the pop-up also helps tell a clear story.
Use predominance to compare multiple attributes within your datasets, and quickly get a map which shows the largest value. This technique is easy to use and can help you visualize new patterns within your datasets.
Learn more about smart mapping and how to use the predominance mapping styles at the ArcGIS Online Help pages:
To see examples of predominance maps, along with additional resources, check out the following maps and story maps:
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