Choropleth map

A choropleth map shades geographic areas according to a statistical value, such as a percentage, rate, or population density. Each area's color represents its value or a range containing that value. It summarizes the area as a whole; it does not show the exact location of each underlying event.

Read the measure before the color

Esri's cartographic guidance describes choropleth maps as a way to compare normalized values, including rates and percentages. Normalization here means relating a count to a suitable base, such as population, so the map is not simply showing where more people live. Esri choropleth documentation.

The legend defines what the shading means. A dark area could represent a high rate, a high percentage, or another value entirely. The color has no meaning independent of the chosen measure, period, and classification rules.

Example

Hypothetical: Area A records 100 incidents among 10,000 residents. Area B records 200 among 40,000 residents. Their rates are 1,000 and 500 incidents per 100,000 residents, respectively.

Area A would fall in the higher-rate category even though Area B has more incidents. A map of raw counts would emphasize a different pattern. The two views answer different questions: relative frequency against a population base and total recorded volume.

Common confusion

A choropleth map differs from a heat map that smooths the concentration of points. Its areas usually follow defined boundaries, and each receives a value for the whole unit. Neither display should be interpreted as a precise risk estimate for every address.

A large colored area also occupies more visual space than a small one, regardless of how many people live there. Visual prominence is not a population measure.

Comparing shaded maps

Check that the legends use the same ranges. If each year's map chooses new breaks between colors, an area's color can change without a comparable change in its value. Read the numbers alongside the shading.

Small denominators can produce unstable rates, so inspect the underlying counts. Also check how the map displays unavailable or suppressed values; those should not be confused with the lowest category.

Boundaries matter because they determine which observations are grouped. A district-wide shade can hide variation within that district. Use the map for the geographic level it represents, and avoid transferring its average to an individual person or property.

Related terms

• Heat map
• Population density
• Normalization
• Small-number volatility

Sources

• Esri: Create and use a choropleth map — Explains shaded statistical maps and normalization. Cited for cartographic principles; the retired software product is not recommended or required.