Historical Research Maps: Population Density

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Maps often tell you a lot about the land, but little about the people who live there. Population density maps attempt to change that by indicating the average number of residents who live in a square mile.

In the map above (which is based on the 1850 federal census), varying shades of purple indicate how many people lived in a square mile, ranging from less than two residents per mile (light purple) to 90-plus (deep purple).

This map is also helpfully labeled with Roman numerals between I (one) and V (five) that indicate the appropriate tier of population density. Dark circles represent the size of individual cities.

In addition to providing general demographic data, these maps can help you envision your ancestors’ communities.

Did they live in a crowded metropolis, or a sprawling small town? What were the major population centers during your ancestors’ region, and how far away from them did your ancestors live? Comparing maps from multiple years can indicate demographic changes or migration patterns.

It’s clear from the map that the Northeast had the highest population density in the country (as it does today), with large sections of New England colored a deep purple and large circles over New York City, Boston and Philadelphia, indicating those cities’ large populations.

The Southern population, however, is much more spread out.

The United States Census Bureau has created population density maps for every federal census since the late 1800s, and you can find and compare them on a convenient landing page. You can access earlier maps like the above at online collections such as the David Rumsey Map Collection.