How to use this map

I have written an introduction to this visualization. Zoom to any county by clicking on it. Clicking on the same county will zoom out. The scales preserve intensity for change over time: in other words, a color represents the same thing for each year on the map. However, the color scales do not necessarily preserve intensity from data field to data field: the darkest color for the total population does not represent the same values as for the enslaved population. The scales for population are logarithmic (with intermediate values) so every second step in the color ramp represents a ten-fold (not a two-fold) increase.

Citation

Map by Lincoln Mullen. Code available on GitHub, licensed MIT. Suggestions and criticism welcome: lincoln@lincolnmullen.com.

If you use this map in your research, I would appreciate a citation. Here is the suggested form:

Lincoln Mullen, "The Spread of U.S. Slavery, 1790–1860," interactive map, https://lincolnmullen.com/projects/slavery/, doi: 10.5281/zenodo.9825.

Minnesota Population Center, *National Historical Geographic Information System: Version 2.0* (Minneapolis, MN: University of Minnesota, 2011), http://www.nhgis.org.

Explanation of Census data

You should also cite the NHGIS:

The U.S. Census data and shapefiles for these maps comes from Minnesota Population Center, National Historical Geographic Information System, version 2.0 (Minneapolis, MN: University of Minnesota, 2011). For a description of the questions asked on the 1790 to 1860 censuses, see Measuring America: The Decennial Censuses From 1790 to 2000 (U.S Census Bureau, 2002). Bear in mind the reason the Census kept statistics on slavery. Slaves were counted in the Census because of the three-fifths compromise in the federal constitution, by which an enslaved person counted as three-fifths of a person when apportioning representation in Congress and direct taxes. I have tried to represent unavailable data on the map, but sometimes in the Census a value of zero actually means that the data has been lost or was never gathered. Treat the Census numbers skeptically: even in the best of circumstances the Census undercounts the population. For example, Harvey Amani Whitfield has shown that Vermont did have slavery, even though no slaves were enumerated in the Census. The numbers are useful chiefly for showing degrees of magnitude. Below are the fields in the NHGIS data that I have used. The total free population was always calculated by subtracting the slave population from the total population.

1790 Census

Slave population: "Race/Slave Status: Persons: Non-White: Slave" (AAQ002)

Total population: (A00AA1790)

Free African American population: "Race/Slave Status: Persons: Non-White: Free" (AAQ001)

1800 Census

Slave population: "Nonwhite Population, Except Indians Not Taxed by Slave Status: Slave" (AAY002)

Total population: (A00AA1800)

Free African American population: "Nonwhite Population, Except Indians Not Taxed by Slave Status: Free" (AAY001)

1810 Census

Slave population: "Nonwhite Population, Except Indians Not Taxed by Slave Status: Slave" (AA7002)

Total population: (A00AA1810)

Free African American population: "Nonwhite Population, Except Indians Not Taxed by Slave Status: Free" (AA7001)

1820 Census

Slave population: sum of "Nonwhite: slave" male and female columns for "Race/Slave Status by Sex" (ABB003 and ABB004)

Total population: (A00AA1820)

Free African American population: sum of "Nonwhite: free" male and female columns for "Race/Slave Status by Sex" (ABB005 and ABB006)

1830 Census

Slave population: sum of "Nonwhite: slave" male and female columns for "Race/Slave Status by Sex" (ABO003 and ABO004)

Total population: (A00AA1830)

Free African American population: sum of "Nonwhite: free" male and female columns for "Race/Slave Status by Sex" (ABO005 and ABO006)

1840 Census

Slave population: "Nonwhite: slave" column for "Race/Slave Status" (ACS003)

Total population: (A00AA1840)

Free African American population: "Nonwhite: free" column for "Race/Slave Status" (ACS002)

1850 Census

Slave population: "Nonwhite: slave" column for "Race/Slave Status" (AE6003)

Total population: (A00AA1840)

Free African American population: "Nonwhite: free" column for "Race/Slave Status" (AE6002)

1860 Census