Students will analyze multiple resources to learn how President Abraham Lincoln and Union forces used a 19th century population density map of enslaved people in the southern United States to help Americans understand the social implications of slavery during the Civil War. This marked the first time in U.S. history that a statistical map was used for this purpose. Suggested Topics: Choropleth maps The Civil War Population density Slavery Statistical maps. Topics: Choropleth maps The Civil War Population density Slavery Statistical maps.