Reference maps versus thematic maps
Most maps fall into one of two broad categories:
Reference maps are meant to show the locations of various phenomena, such a countries, cities, rivers, etc. Chances are the maps you use most in your daily life are reference maps—street maps (e.g., Google Maps) that help you see where things are. Other common reference maps emphasize physical landscapes—think topographic maps or maps of a national park.
Thematic maps are meant not simply to show locations, but rather to show attributes or statistics about places, spatial patterns of those attributes, and relationships between places. For example, while a reference map might show the locations of cities, a thematic map might also represent the population of those cities. It’s the difference between mapping places and mapping data. This site is about thematic maps, describing some of the different types and basic principles.
Genereally speaking, thematic cartography is about symbolizing data in different ways. We do this through the use of several visual variables of grahics such as size, color, and shape. The exact way in which data should be symbolized depends on the nature of the data (is it a count of things? is it a categorical description?), as well as the nature of the geography (is it an individual point? is it a large area?). In these pages we aim to describe some of the common ways to represent thematic map data, along with some tips and best practices.