Smart Mapping: Dot Density

Dot density allows us to visualize numeric data in a unique way. Instead of entirely filling a polygon with a thematic color, dots on the map represent counts from your data. For example, each dot could represent a household, or a person. This mapping method is popular when mapping demographic and socioeconomic patterns, but can be used to map any type of numeric/quantitative data.

When you want to map a polygon layer using a count of something like population, it’s tempting to shade the entire polygon based its count. Why? The size of the polygons dramatically affects the patterns on the map, much more so than the actual data you are mapping. What’s needed are map styles that fairly and evenly represent counts regardless of the size of the polygons in that layer.

For example, if we want to map the population in the US, one method we could use is to show the population using size. While the size mapping style highlights major cities, it lacks the ability to show nuances in the rural and suburban parts of the country. Dot density allows us to fill each polygon with dots that represent a count, in this case population, providing a representation of the population density.

Swipe the maps below to compare the two methods:

We can use dot density to represent a single attribute like the population example above, but it can also be used to compare related attributes within your data.

For example, if I want to compare housing units by their occupation type, dot density can help me do so. There are three types of occupation types: owner-occupied housing units, renter-occupied housing units, and vacant housing units. These exist as three numeric attributes within my table:

Dot density allows me to represent housing units on my map, while allowing me to also compare where each housing type exists. If we explore a city such as Miami, Florida, we can quickly learn a few things:

• The city center contains dense housing
• There are more renters in the city center
• People are more likely to own if they live outside of the city center
• Vacant housing units are prevalent along the barrier islands
• Many areas contain a mix of all housing types

Dot density gives us an easy way to visualize and compare things within our maps. It helps communicate density, overlapping patterns, and more.

Dot density is a powerful way to show where patterns overlap (or don't overlap) in our data, while humanizing a number into something like a person or housing unit. But how can we effectively create a map like this?

Smart mapping helps you create beautiful and informative maps, quickly. It is a capability, that by default, sets the cartography of your map based on the significant values within your data. When working in ArcGIS Online and ArcGIS Enterprise, choose the Change Style pane from the right toolbar to see which smart mapping drawing styles can work with your layer. If you have a polygon layer and choose a numeric attribute like a population count, total expenditures value, or other total, you’ll see the option for Dot Density.

To start, you'll need a polygon layer which has one or more numeric attributes you want to compare. Let's walk through an example:

In many cases, a polygon may contain a mix of multiple categories. By default, the option "blend overlapping colors" is enabled. What this does is blend the colors to avoid one color covering the other. This blending technique helps display areas that have roughly equal counts of two categories brightly in the blended color.

The dot density color ramps were designed to blend, so blue and yellow will make green, red and yellow make orange, etc.

Now that you have the tools for success, share it with the GIS community! Tag your maps on social media with #smartmapping and #ArcGIS Online to show others what is possible.