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Mapmaking Innovation Aids Pro-Union Cause, 1861
Slave Density Map
Click Image to View Full Size

It isn’t often that a map can visually display a moral issue facing a divided nation. Nearly 150 years ago, the U.S. Coast Survey achieved that landmark representation. The map, showing the distribution of the slave population in the Southern states, is included in Coast Survey’s new special collection of maps and charts that were produced during the Civil War.

One of the collection’s most important maps is the “Map showing the distribution of the slave population of the southern states and the United States.” According to both contemporary sources and historians, President Abraham Lincoln used the map to connect military forays to his policy of emancipation.

“These maps were among the first to use shading to represent the human population,” explains Capt. Albert Theberge (Ret., NOAA Corps), the chief of reference for the NOAA Central Library. “They are prime examples of how Coast Survey science aided the Union cause during the Civil War.”

Created in September 1861, the map graphically displays the density of the slave population in the Southern states, based on statistics from the 8th Census, which was supervised by Joseph Camp Griffith Kennedy. Kennedy wanted to include slaves by name in the U.S. Census Report, but Congress refused. Alexander D. Bache, the U.S. Coast Survey Superintendent, allowed his staff to undertake such a map. It was created under Edwin Hergesheimer, a cartographer with U.S. Coast Survey’s drawing division.

The development of this map was revolutionary for several reasons. In addition to initiating a trend of statistical cartography, its thematic display of “moral statistics” affected political change. Northern audiences were able to see that the first states to secede were those with the most slaves. By the use of shading to represent the human population, the darkest areas of the maps show the highest density slave populations, and the order of secession corresponds closely to the shade densities of the map. Even more to the point, a table in the map corner shows the number of slaves in each state, and the proportion of slaves to the total population. The order of the list, from highest density to low, again corresponds closely to the order of secession.

According to artist Francis Bicknell Carpenter, who painted First Reading of the Emancipation Proclamation of President Lincolnin 1864, President Lincoln frequently consulted this map in considering the relationship between emancipation and military strategy. Carpenter observed that Lincoln would look at the map and send his armies to free blacks in some of the highest density areas to destabilize Southern order.

The NOAA connection to the map had been lost over the decades. John Cloud, Ph.D., a historian who was recovering significant Coast Survey cartographic products in NOAA’s Climate Database Modernization Program, recently discovered the connection with Edwin Hergesheimer and U.S. Coast Survey. Historian Susan Schulten made important historical contributions in connecting the map to Francis Bicknell Carpenter's painting and, in extension, to President Lincoln's strategy.

To access a high resolution image of the map, go to Charting a More Perfect Union, enter keyword slave, and either preview the image or add to your cart. The image is free.

This year the nation observes the 150th anniversary of the Civil War. To help people today envision the landscapes and coastal hazards that were important to military strategies, NOAA’s Office of Coast Survey has unveiled “Charting a More Perfect Union,” the collection of maps, charts, and sketches produced during the war by U.S. Coast Survey, NOAA’s predecessor organization.

This project is supported by the NOAA Preserve America Initiative, part of Preserve America, a federal initiative to preserve, protect and promote our nation's rich heritage.

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