Univariate choropleth maps

http://www.agcensus.usda.gov/Publications/2002/Ag_Atlas_Maps/Crops_and_Plants/index.asp

This map shows a univariate chloropleth map portraying acres of harvested corn for grain in 2002. Univariate Chloropleth maps show one set of data over an area. On this map we see the one set of data--number of acres harvested--over the entire U.S.