Choropleth vs Heat Map vs Proportional-Symbol Map

Use a choropleth when a comparable value belongs to predefined areas, a heat map when you want to show the concentration of many point observations, and a proportional-symbol map when the magnitude of a value belongs to a particular location or place.

They can all make “high” and “low” patterns visible, but they encode fundamentally different geographic claims.

Map type · Geographic support · Main visual variable · Typical question

Choropleth — predefined polygons — colour/lightness — Which areas have higher rates or percentages?
Heat map — many points, summarised into a surface — intensity — Where are point observations concentrated?
Proportional symbols — locations or representative place positions — symbol area — Which places have larger totals?

Choropleth: compare area-level measures

A choropleth colours polygons such as countries, districts or census tracts according to a value, which should normally be meaningful for comparison across those areas: a rate, percentage, density, index or another normalised measure.

A choropleth inherits the geography of the boundary system. It is therefore appropriate when the data themselves are reported for those areas, but it can also make the boundaries feel more substantive than they are.

Raw totals often need another representation because larger-population areas tend to have larger counts. What Is a Choropleth Map? covers the method in more detail.

Heat map: summarise point concentration

A geographic heat map usually starts from point observations and creates a continuous-looking intensity field around them, with nearby points reinforcing one another to produce “hotter” areas.

The result depends on parameters such as radius or bandwidth: a wider radius produces smoother, broader patterns, while a narrower radius preserves more local variation. Weighted heat maps can also give some points more influence than others.

A heat map is therefore not just a different style for a point layer. It is a spatial summary whose appearance depends on analytical choices. It is useful for patterns such as incident concentration or activity intensity when individual point identity is secondary.

Proportional symbols: show magnitude at locations

A proportional-symbol map places symbols—often circles—at locations and scales their area according to a numeric value. This is often a better choice for totals such as city population, port throughput or cases by facility because the value can remain attached to a place without colouring an entire surrounding polygon: the symbol's position identifies the place, while its area communicates magnitude.

Symbols can overlap in dense regions, so placement, transparency and scale limits still require judgement.

The same dataset can support different claims

Suppose you have disease notifications by district plus the locations of individual clinics.

A choropleth of incidence per 100,000 residents asks which districts have higher rates, a heat map of notification points asks where recorded events are spatially concentrated, and proportional symbols showing total cases at clinic locations ask which facilities reported more cases.

Those maps are not interchangeable visualisations of one fact. They answer different questions from different spatial supports.

Common mistakes

Do not use a heat map when exact locations matter. Smoothing can hide the identity of individual observations and imply continuity between them.

Do not use a choropleth simply because you have polygons. If the mapped variable is a raw count, the area may mostly reproduce differences in population or exposure.

Do not size proportional symbols by radius when the data value is meant to be proportional to symbol magnitude. Readers perceive the whole symbol area; doubling the radius quadruples the area.

Which should you choose?

Choose a choropleth when the statistic belongs to areas and comparison across those areas is the task. Choose a heat map when point concentration is the task and individual locations can be summarised. Choose proportional symbols when numeric magnitude belongs to places or locations and should remain visually independent of polygon size.

If two choices still seem plausible, return to the question: What does one mark on this map represent? The answer usually reveals whether the map should be an area comparison, a concentration surface or a magnitude-at-location display.

The choice changes what “high” means

The word high means something different in each map. In a choropleth, a dark area may mean a high rate across an administrative unit; in a heat map, a bright patch means many point observations influence that part of the surface; and in a proportional-symbol map, a large circle means one location or place has a large numeric value.

This distinction is useful when reviewing a draft. If you cannot finish the sentence “a high value here means…”, the representation may not yet match the question. It also helps prevent a common mistake in dashboards: switching between map types while keeping the same title, as though the maps were interchangeable views of one concept.

What happens when the data are aggregated first

The source data may already constrain the choice. If individual events have been aggregated to districts and the original point locations are no longer available, a true event-density heat map cannot be reconstructed from the district totals. Conversely, if the only data are event points, a district choropleth requires an explicit aggregation step and a defensible denominator if the result is meant to represent a rate.

Map type selection therefore belongs partly to data preparation. A visually attractive representation cannot recover spatial detail or statistical meaning that the source data do not contain.