Disunion follows the Civil War as it unfolded.

The 1860 Census was the last time the federal government took a count of the South’s vast slave population. Several months later, the United States Coast Survey—arguably the most important scientific agency in the nation at the time—issued two maps of slavery that drew on the Census data, the first of Virginia and the second of Southern states as a whole. Though many Americans knew that dependence on slave labor varied throughout the South, these maps uniquely captured the complexity of the institution and struck a chord with a public hungry for information about the rebellion.

The map uses what was then a new technique in statistical cartography: Each county not only displays its slave population numerically, but is shaded (the darker the shading, the higher the number of slaves) to visualize the concentration of slavery across the region. The counties along the Mississippi River and in coastal South Carolina are almost black, while Kentucky and the Appalachians are nearly white.

Interactive Map A Map of American Slavery Read the author’s detailed descriptions of one of the most important maps of the Civil War.

The map reaffirmed the belief of many in the Union that secession was driven not by a notion of “state rights,” but by the defense of a labor system. A table at the lower edge of the map measured each state’s slave population, and contemporaries would have immediately noticed that this corresponded closely to the order of secession. South Carolina, which led the rebellion, was one of two states which enslaved a majority of its population, a fact starkly represented on the map.

Conversely, the map illustrated the degree to which entire regions—like eastern Tennessee and western Virginia—were virtually devoid of slavery, and thus potential sources of resistance to secession. Such a map might have reinforced President Abraham Lincoln’s belief that secession was animated by a minority and could be reversed if Southern Unionists were given sufficient time and support.

The map quickly caught the public’s attention, and was reproduced throughout the war. Its banner headline, “for the benefit of the sick and wounded soldiers,” also became the slogan of the Union’s most important homefront organization, the United States Sanitary Commission. The map gave a clear picture of what the Union was up against, and allowed Northerners to follow the progress of the war and the liberation of slave populations.

We don’t know when Lincoln first encountered the Coast Survey’s map of slavery. But he became so taken with it that Francis Bicknell Carpenter included it in the lower right corner of his painting, “President Lincoln Reading the Emancipation Proclamation to His Cabinet.” Carpenter spent the first six months of 1864 in the White House preparing the portrait, and on more than one occasion found Lincoln poring over the map. Though the president had abundant maps at his disposal, only this one allowed him to focus on the Confederacy’s greatest asset: its labor system. After January 1, 1863—when the Emancipation Proclamation became law—the president could use the map to follow Union troops as they liberated slaves and destabilized the rebellion. Lincoln was enthusiastic about Carpenter’s finished portrait, and singled out the map as one of its most notable details.

U.S. Senate Collection

(Slavery also informed the painting in another way. Carpenter arranged the Cabinet according to his perception of their sentiment regarding emancipation: its two leading proponents, Secretary of the Treasury Salmon P. Chase—standing—and Secretary of War Edwin Stanton, are to Lincoln’s right, while Secretary of State Seward sits in the foreground. To Lincoln’s immediate left are the secretary of the navy, the secretary of the interior and the postmaster general standing to the rear, while Attorney General Edward Bates sits at the far right of the portrait. Lincoln sits at the center, as Carpenter wrote, “nearest that representing the radical, but the uniting point of both.” A copy of the anti-slavery New York Tribune lies at Stanton’s feet, while a portrait of Simon Cameron—the prior secretary of war who urged emancipation early in the conflict—is visible beyond Stanton’s head. The map lying across the table behind Seward is the Coast Survey’s 1863 “Map of the State of Virginia,” which included both population statistics and concentric rings around Richmond to guide Union strategy.)

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It may seem odd that the Coast Survey—originally responsible for detailing the nation’s coastlines and rivers—produced a map of slavery in the south. Yet over the preceding two decades its superintendent, Alexander Dallas Bache, had skillfully widened the Survey’s work and made it a hub of mapmaking innovation. The Survey experimented with several new methods of cartographic representation, including the use of shading to represent the human population. As early as 1858 Bache had directed the Survey to produce maps of the rivers and coasts of the South, in anticipation of a conflict. But the 1861 map was in a class by itself: a landmark cartographic achievement, a popular propaganda tool, and an eminently practical instrument of military policy. No wonder Lincoln liked it.

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Susan Schulten is a history professor at the University of Denver and the author of “The Geographical Imagination in America, 1880-1950.” She is writing a book about the rise of thematic mapping in the United States.