This map, showing “the distribution of the slave population of the southern states and the United States,” was created in September 1861 based on statistics from the eighth Census. Says the NOAA:

“President Lincoln frequently consulted this map in considering the relationship between emancipation and military strategy… Lincoln would look at the map and send his armies to free blacks in some of the highest density areas in order to destabilize Southern order. The Emancipation Proclamation became law on Jan. 1, 1863.”

19th century map (1861) showing slaves population distribution in the southern states and the united states

For a closer look, click here.