Choropleth Map

Source: http://www.agcensus.usda.gov/Publications/2002/Ag_Atlas_Maps/Crops_and_Plants/Field_Crops_Harvested/Wheat/All%20Wheat%20for%20Grain,%20Harvested%20Acres-chor.gif

This is another type of thematic map, but it uses color and shading to represent something at a physical location. The map above is for all wheat for grain harvested by acre in 2002. The color varies from pale yellow-green to dark green and sections are divided by county. The darker color showing higher density. Looking at the map it becomes clear that the mid-west and especially the northern boarders of the country are the wheat producing mecca of the United States.