Thematic maps and map projections

A thematic map shows information about one topic.
Choropleth: areas shaded darker for higher values (population density by county).
Isoline: lines joining places with the same value. Contours join equal height; isobars join equal air pressure; isotherms join equal temperature.
Dot distribution: each dot stands for a set number of things, such as 1 dot = 1,000 people.
Proportional symbols: bigger circles or bars for bigger values.
Flow lines: arrows whose width shows how much is moving, such as migrants or traffic.

The Earth is a sphere, so it cannot be flattened without stretching or tearing. A way of flattening it is a map projection. Every projection distorts area, shape, distance or direction.
The Mercator projection (1569, by Gerardus Mercator) keeps directions true, which made it useful for sailors. But it hugely enlarges places near the poles.
On a Mercator map Greenland looks about as big as Africa, yet Africa is about 14 times larger.

Equal-area projections, such as the Gall-Peters, show the true size of countries, but stretch their shapes.
Compromise projections, such as the Robinson or Winkel Tripel, balance the errors and are common in atlases.
Only a globe shows sizes, shapes and directions correctly all at once.