Edwin Hergesheimer, Map Showing the Distribution of the Slave Population of the Southern States of the United States (1861)

In 1860, the U.S. Census estimated the population to be over 31 million people, of which nearly 4 million were enslaved. This was a shocking statistic; by comparison, in 1790, the number of enslaved people had been only 700,000. The issue of slavery divided the nation, and six months into the American Civil War, U.S. Coast Survey cartographer Edwin Hergesheimer captured the division through this landmark map, showing the distribution of the enslaved population among the Southern states. Hergesheimer used new choropleth mapping techniques, shading the map according to statistical data--in this case, the percentage of the population registered as slaves in each state's counties according to the U.S. census. As discussed in the Smithsonian collection, Great Maps: The World's Masterpieces Explored and Explained, Hergesheimer's map reveals high concentrations of slavery in areas where tobacco, sugar, and cotton plantation farming were most profitable, explaining the secession crisis as being driven by economic fear of abolishing slavery. The map also became important in Abraham Lincoln's military strategy as he would consult it when considering where Union troops might destabilize the Southern order in areas of high slave populations. Painter Francis Bicknell Carpenter captured the importance of the map to Lincoln in his 1864 painting, “First Reading of the Emancipation Proclamation of President Lincoln,” where you can see the map in the corner.