This is a version, published in Harper's Weekly, of the famous "Slave Map" studied and relied on by Lincoln in prosecuting the Civil War. The map shows the population density of slaves at each point in the Confederacy, and while there is no key on the map itself, an accompanying article provides detail on percentages. As the article points out, "the density of slave population presents a proportionate abundance in the product of cotton," as well as "the production of sugar" along the Gulf Coast.
Lincoln's version of the slave map had been produced by Coast Survey. It was substantially larger than the Harper's map and clearly showed county borders, making it suitable for Lincoln to use in following advances on the ground. "Lincoln kept it close at hand and consulted it repeatedly;" he was "frequently engrossed" in it. Schulten 2010, 140, 142. When the portraitist Francis Carpenter borrowed it to incorporate it into his famous First Reading of the Emancipation Proclamation, the President promptly and personally came to reclaim it, "put on his spectacles, and, taking it up, walked to the window; and sitting down upon a trunk began to pore over it very earnestly," commenting that General Kilpatrick was "close upon" a county "where slaves are thickest." Ibid. 142.