Choropleth, Dot, and Graduated Symbol Maps

A choropleth map shades entire areas, like counties or states, based on a single summary number for each one: a rate, a percentage, or an average. The mapmaker sorts all the area's values into a few groups, called classes, then assigns each class a color shade, usually darker for higher values. A spreadsheet of county incomes becomes a patchwork of light and dark counties at a glance. A dot map instead scatters individual dots across the area, where each dot stands for a fixed quantity, so density of dots shows density of the real thing on the ground. A graduated symbol map puts a symbol, often a circle, at an exact point location and scales the symbol's size to match the value there, so a city with triple the population gets a visibly bigger circle, not a bigger shaded region.

The choice of map type depends on what the number actually represents. Raw counts tied to specific places, like bushels harvested at individual farms, fit dot maps or graduated symbols, since each unit or location is a real point. Rates and percentages that only make sense per area, like the share of a workforce in agriculture, fit choropleth maps, because that value has already been normalized to the county's size and can be compared fairly across counties of different populations.

This matters because the same table of numbers can look crowded or empty depending entirely on how it is mapped and how classes are drawn, which is why this topic sits right after projection distortion: both are reminders that every map is a choice, not a neutral mirror of the world.