Anderson (1970) published this map of Illinois showing the distribution of forest and prairie around 1820. It was created using GIS and the original Government Land Office (GLO) maps from the early 1800s. It shows the breakout of forest, prairie, and water and can be used to created acreages by county and by region. Although the map is very useful, it does not relay information about the species composition of forests nor is its resolution high enough to locate small (<500 acres) parcels, such as hill prairies or forest groves.