Density Map

$image\density.gif$ Density maps use randomly positioned symbols to represent data values for areas on a map. On a density map, each symbol represents a set number of data units, so the number of symbols drawn in an area is proportional to the data value associated with that area. Areas with more symbols have higher associated data values. This type of map does not use classes like the hatch map and can show a more continuous distribution.

Density maps are effective at displaying data because the density of symbols is in relation to the size of an area. For example, consider a small area and a large area that both have the same associated data value. Although the two areas display the same number of symbols, the symbols are more crowded in the small area and therefore indicate the higher relative density of data for that area.

Creating and Editing a Density Map

Select a layer with data linked boundary objects and click the Map | Create Map | Density command . Edit density maps in the Property Manager.

See Creating and Editing Thematic Maps for information on creating a map, changing a map to another map type, and changing map properties.

Density Map Properties

The Property Manager for a density map contains General, Symbol, Data Labels, Info, and Map pages. Click the preceding hyperlinks for information on properties in each of the property manager pages.

Map Page

The Property Manager Map page contains properties unique to density maps.

Select the density method and options in the Map page of the Property Manager.

Symbol Values

The Density group lets you specify the relative number of dots to draw on the map. There are two methods you can use to create your density map. The Dot ratio method assigns a data value to each dot, or defines a set number of dots for the maximum data value. The Dot sum method defines a number of dots that represent the sum total of the variable. This method provides a simple way to make each dot represent a percentage of the data. For example, if the total number of dots is set to 100, each dot represents one percent of the variable.

Density settings:

Dots for max data is the number of dots that appear in the area with the largest data value for the Dot ratio method.

The Dot value field displays the number of data units represented by a single dot for the Dot ratio method. This field is linked to the Dots for max data value. When you change the value of either field, the other is updated accordingly. The Dot value field is calculated by the maximum data value divided by the Dots for max data value.

You can set the Dot value to any number, but MapViewer might recalculate the Dot value slightly to ensure the Dots for max data field is an integer. Also, the Dot value can be set to be larger than the data minimum. In this case, the area with the data minimum does not show any dots on the density map. Although, because of rounding, a single dot might appear in an area with data slightly below the Dot value.

The Sum of dots value defines the total number of dots used to represent the sum of the variable values for the Dot sum method.

Random Positioning

Check the Position dots randomly box to randomly position the dots each time the map is redrawn.