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Dot density

Dot density symbology is one way to represent quantities within polygons on a map. With dot density symbology, the data you are symbolizing is not classified. Instead, quantitative values for one or more fields are represented as a collection of point symbols (typically solid circles or dots) within each polygon. Each dot represents a constant number of things, or people, or other quantifiable phenomena. The dots are equally sized, even when multiple fields are symbolized together within a layer.

To mimic a natural data distribution, try symbolizing a fine-grained dataset like United States counties with dot density symbology with a completely transparent polygon symbol. Overlay this layer over a coarser-grained categorization layer, like United States states symbolized with single-symbol symbology. This gives a better indication of where people actually live than if the dot density symbology were applied to the states features instead. An example is shown below.

2012 population per U.S. county drawn with dot density symbology in gray, shown above U.S. states, shown in light orange

2012 population per U.S. county drawn with dot density symbology in gray, shown above U.S. states, shown in light orange

When creating a dot density layer, you specify how many features each dot represents, and then specify the size of the dots. You may need to try several combinations of amount and size to see which one best shows the pattern. In general, you should choose values that ensure that the dots are not so close as to form solid areas that obscure the patterns, or so far apart as to make the variations in density hard to see. In most cases, you'll only map one field using a dot density map. In special cases, you may want to compare distributions of different types and may choose to map two or three fields.

To obtain a reasonable density of dots, the scale at which you view the layer when you apply the dot density symbology affects the default values that are chosen. You can then modify these values. Experiment with different dot values and dot sizes to get an appropriate density of features. Ideally the dots don't coalescence except in the densest areas. Conversely, you don't want the dots so small or sparse that it is difficult to discern spatial patterns in the data. The location of the points stays static as you zoom in and out. As with other symbology methods, use a reference scale to make the symbols scale relative to the geometry.

You cannot apply dot density to a layer in the 3D category of a scene, and you cannot drag a layer drawn with dot density symbology into the 3D layer category. For this reason, you cannot specify real-world units for the symbol size of the dots.

Format the dots

Typically dot density maps use small solid circular dots. This is because the point of the visualization is to see density and distributions. You can choose a different shape—any supported point symbol—but understand that you may compromise the map's legibility and understandability, which is based on the density and distribution of the dots. Make sure you have a good reason for picking something other than circles.

Unlike other symbology methods, the size of the symbols in dot density symbology is set by the dot density symbology method itself, using the Dot Size property. You can customize the color and shape of the dot how you like, but the size you set in the Format Point Symbol pane will be ignored and overridden by the Dot Size property in the Symbology pane for the dot density symbol method.

The symbology is drawn as point symbols distributed within polygon features. The polygon remains the feature type of the layer. When you select or identify features in the layer, it is polygons that you will interact with. You will not be able to select or query the individual points. Neither the dots nor the polygons participate in symbol layer drawing.

Format the background polygons

The dots are randomly, yet evenly, distributed throughout the polygon feature. You can specify a symbol for the polygon itself as well, which by default is a hollow (transparent) polygon with a gray outline.

You can specify multiple fields to show comparisons between the attributes. In the example below, the number of people who own their homes (shown in gray) is compared to the number of people who rent their homes (shown in dark orange.)

Dot density symbology with two fields

Dot density symbology with two fields. The U.S. population per county, with those who own homes shown in gray, compared to those who rent, shown in dark orange.

Draw a layer with dot density symbology

  1. Select a feature layer in the Contents pane.
  2. On the Appearance tab, in the Drawing group, click Symbology and click Dot Density.

    The Symbology pane appears.

  3. In the Symbology pane, choose one or more numeric fields to be mapped.
  4. Choose a color scheme that assigns dot colors for each of the fields displayed as part of the dot density layer. Optionally click the symbol for each field to modify.

    Only the shape and color of the symbol you set in the Format Point Symbol pane is respected. The size is ignored. Dot size is dictated by the Dot Size property in the Symbology pane.

  5. Change the Dot Size and the Dot Value as necessary to achieve appropriate density and distribution for the data.
  6. Optionally click the Background symbol to modify the appearance of the polygon symbol used to display the underlying polygon features.

Tip:

You can change the dot symbol directly by clicking it in the Contents pane, or right-click the symbol to change just its color. You can change the background symbol directly from the Contents pane in the same way.

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