Use a choropleth for values attached to administrative regions
For showing geospatial values by administrative region, prefer a choropleth over a symbol map to improve fidelity and mitigate treating region data as exact locations for map readers.
- purpose:select
- basis:heuristic
- chart:choropleth:use
- chart:map:avoid
- data:geospatial
- lever:chart-family
- quality:fidelity
advice
Region fills for region data
Fill administrative regions when the data is reported for regions rather than for exact locations. For example, map turnout or crime rates by municipality or province with a choropleth instead of placing symbols as if each value belonged to a single point.
reason
Why filled regions match region-level data
Region-level data belongs to boundaries, not to isolated points. A choropleth keeps the geographic unit of the data visible.
Mechanism: Coloring whole regions matches how the values are stored and read, so the map does not imply false point precision.
Evidence: Choropleth maps are recommended for data available in administrative regions such as municipalities or provinces, while symbol maps are reserved for lots of specific locations (Muth, 2025).
context
Use when each value belongs to a named region
- User Goal: Show a geographic pattern across administrative areas.
- Task: Compare region-level values on a map.
- Data: Geospatial values attached to municipalities, provinces, or similar regions.
- Chart Setting: A map where the data is aggregated by boundary.
- Audience: Map readers looking for regional patterns.
- Success Criterion: The map makes the relevant regions and their values immediately visible.
exceptions
Do not use when the data is a set of exact places
Break it when: The dataset consists of many specific locations rather than region-level aggregates. Why: Symbol maps are the right choice for mapping lots of exact places.
costs
Costs of switching to a choropleth
Sacrifice: You give up point-by-point location markers.
Risk: If the data is actually point-based, a choropleth implies an aggregation the data does not have.
Mitigation: Use choropleths only for values that belong to administrative regions.
mistakes
Common failure with regional map data
Mistake: Use point symbols for values that are only available by municipality, province, or another administrative region. Why it fails: The map stops matching the geographic unit of the data.
check
Check whether the geographic unit is a region rather than a point
Failure Sign: Each data row is an administrative area, not an exact place.
Quick Check: Compare a choropleth draft with a symbol-map draft; if the symbol map only stands in for whole regions, keep the choropleth.
Stronger Test: Ask whether the data can be joined to boundaries directly without inventing exact point locations.
fix
Fix the region-level map
- Join the values to administrative boundaries.
- Encode the values by filling each region lighter or darker.
- Remove point symbols that imply an exact location the data does not provide.