dotdensity

Dot density is a cartographic technique that represents quantities by placing evenly sized dots within geographic areas. Each dot corresponds to a fixed quantity of the variable being mapped, such as 100 people or 1 household. The dots are placed inside the boundaries of the geographic unit and may be drawn randomly or in a stratified way to reflect known sub-areas.

Process: For each area, compute the number of dots as value divided by dot value. Some maps reserve a portion to represent missing or unknown data. Dots can be colored or shaped to encode additional categories (e.g., race, age group, housing type).

Purpose and interpretation: The technique highlights spatial patterns, clustering, and gaps that may be obscured in aggregated polygon maps. It provides a visual impression of density and distribution rather than precise counts at fine scale.

Limitations: Dot placement randomness introduces sampling error; the choice of dot value affects perceived density and comparability; small areas may show little detail; multiple attributes require layered color schemes that can be hard to read. Privacy considerations apply when dots imply individual people.

Variations and related methods: Multicolor dot density uses different colors for subgroups. Related methods include kernel density estimation and point density mapping, which use continuous surfaces rather than discrete dots; choropleth maps are a common alternative that show aggregated values by area rather than distribution within areas.

History and usage: Dot density has been used in demographic mapping, urban planning, and social science research to communicate distributions in a visually intuitive way.