Visualizing slavery from 1860

From the NYT comes a great article about an early map from the US Coast Survey (creators of those lovely coastal charts from the late 19th century that adorn many of my walls) that shows slavery in the southern US, based on the 1860 census. The map used novel cartographic techniques for the day and was a masterful piece of public outreach: it was important in convincing the Union public that the civil war was about slavery, and not just state's rights. Map here.

From the article: 

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 (legend at left). The counties along the Mississippi River and in coastal South Carolina are almost black, while Kentucky and the Appalachians are nearly white.