In Kinds of Maps you met reference maps (where things are) and thematic maps (one topic, such as rainfall). Once you have geographic data, you have to choose how to map it. The right choice depends on what the data measures.
| Map | How it shows data | Best for |
|---|---|---|
| Choropleth | Shades whole areas (states, countries) by a value | Rates and percentages: people per km², percent with internet |
| Dot distribution | One dot stands for a number of things | Where things are: farms, schools, cases of a disease |
| Graduated symbol | Bigger symbols for bigger amounts | Totals at points: city populations, port traffic |
| Isoline | Lines join places with equal values | Smooth surfaces: elevation, temperature, rainfall |
| Cartogram | Resizes each place by a value | Comparing totals: population, wealth |
Key idea
Choose the map from the data: rates on a choropleth, locations as dots, totals as graduated symbols or a cartogram, continuous values as isolines.
Why it works
On a choropleth map the eye reads area: big, dark regions look important. If you shade by totals, large regions get dark just because they are large and hold more people. A rate divides by area (or by population), which removes that size effect, so the shading compares places fairly.
Worked examples
Worked example: Matching data to maps
Choose a map for each: (a) the number of passengers at 30 airports; (b) the percent of adults with a college degree in each state; (c) average January temperature.
- (a) Totals at points: graduated symbols.
- (b) A percentage for each area: a choropleth.
- (c) A value that changes smoothly across the land: isolines.
Answer: (a) graduated symbols, (b) choropleth, (c) isolines.
Worked example: Fixing a map
A student shades U.S. states by total number of farms. Texas is darkest. What is the problem and the fix?
- Texas is huge, so it has many farms partly because it has so much land.
- Shading by totals makes big states look important.
- Fix: shade by a rate (farms per 1,000 km², or percent of land in farms), or show totals with graduated symbols.
Answer: Use a rate on the choropleth, or graduated symbols for the totals.
Common mistake
The most common thematic-map mistake is a choropleth of raw totals. Check: does the legend have "per" or "percent" in it? If not, and the map shades areas, it probably should use a rate instead.