The historian and cartographer Bill Rankin visualizes the practice with a fresh approach.

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. Here’s Rankin on what his novel approach achieves:

Cities usually had fewer slaves, proportionally, than their surrounding counties, but this is invisible on standard maps. Adding this data shows the overwhelming predominance of slaves along the South Carolina coast, in contrast to Charleston; it also shows how distinctive New Orleans was from other southern cities. These techniques don't solve all problems (especially in sparsely populated areas), but they substantially refocus the visual argument of the maps — away from arbitrary jurisdictions and toward human beings.

Here are Rankin’s maps in gif form to show the change in the slave population with each successive decade.

Rankin also created a “peak slavery” map showing where slavery expanded and contracted over time. In this map, the size of the dots varies according to the highest number of slaves in that cell, and their color depicts the year this number peaked:

Here’s the takeaway, according to Rankin:

The gradual decline of slavery in the north was matched by its explosive expansion in the south, especially with the transition from the longstanding slave areas along the Atlantic coast to the new cotton plantations of the Lower South. And although the Civil War by no means ended the struggle for racial equality, it marked a dramatic turning point; antebellum slavery was a robust institution that showed no signs of decline.

Of course, the legacy of slavery in the U.S. remains, shaping the politics and economies of its cities to this day.

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