Although it was abolished in 1865, slavery in the U.S. is still being mapped by cartographers looking for fresh approaches to the topic. The latest effort comes from the historian and cartographer Bill Rankin.

Rankin’s new maps provide snapshots of U.S. slave populations from 1790 to 1870 in 10-year intervals. But his methodology is a departure from that of previous cartographers in that it doesn’t take counties as the smallest units of analysis.

He explains the problems with examining slavery at the county-level on his website:

Visually, it is tough to compare small and large counties; the constant reorganization of boundaries in the west means that comparisons across decades are tricky, too. And like all maps that shade large areas using a single color, typical maps of slavery make it impossible to see population density and demographic breakdown at the same time. (Should a county with 10,000 people and 1,000 slaves appear the same as one that has 100 people and 10 slaves?) 

Instead, Rankin (whose previous works should be familiar to CityLab readers) measures the population within uniform cells that are 250 square meters in area, and represents each as a single dot. These dots are color-coded based on the percentage of slaves within the total population in that cell. To supplement the county-level numbers, Rankin added population data from urban areas during this time. ...