Not every map has the same job. Before you read a map, check its title and key to see what kind it is. This builds on Why Flat Maps Stretch and the map keys you used in Maps and Places.
Key idea
Political maps show borders, countries and cities. Physical maps show landforms and water: mountains, rivers, deserts. Thematic maps show one topic, such as rainfall, population or language.
Why it works
A map that tries to show everything becomes a mess. Each kind leaves things out so its main idea stands out. A thematic map often uses a choropleth: each area is shaded by one number, using a color scale explained in the key. It makes patterns jump out, but it gives only one value for each whole area.
Worked example: Which kind?
A map shows where earthquakes happened last year as red dots. What kind of map is it?
- It is about one topic: earthquakes.
- It is not mainly about borders or landforms.
Answer: A thematic map.
Worked example: Reading a choropleth
Suppose a choropleth shades each country by its yearly rainfall, and Brazil is in the wettest class. Does every part of Brazil get that much rain?
- The shade stands for one number for the whole country, such as its average.
- It cannot show differences inside a country.
- In fact the Amazon in the north is very wet, while parts of the northeast are dry.
Answer: No. A choropleth gives one value per area, not what happens inside it.
Common mistake
Reading a choropleth as if it shows where things are inside each area. A country in the top class for population may have most of its people in a few cities and almost none in its deserts. For where things are inside, look for a dot map or a map of smaller areas.