http://www.businessinsider.com/heres-the-map-that-lincoln-used-to-see-the-reach-of-slavery-2013-9

"This map, made by the U.S. Coast Survey in 1861 using census data from
1860, shows the relative prevalence of slavery in Southern counties that
year. "

The map is zoomable, so the counties can be easily read.

A powerful visual.

/DDD