When the 2020 Census was released in August 2021, maps showing population change across the country’s 50 states and over 3,100 counties were some of the first analysis produced with the new data. The stable boundaries of these legal geographic entities make creating such maps relatively straight-forward.
Maps showing more detailed subcounty population change between decades are less common because revisions are made to the Census Bureau’s statistical areas called “tracts”.
Fortunately, there are tools that can be used to overcome these limitations and the Tennessee State Data Center used them to produce an interactive map showing population change from 2010 to 2020 across the country.
This map of subcounty population change reveals interesting patterns not evident in the more often used county-level maps. These include:
- Growth centers in densely populated urban cores
- Areas of population increase and decrease within a single county
- Multi-county regions of population growth at the suburban edge
- Cross-border clusters with population loss
“Crosswalking” Census Data
In the 2020 Census, data for the 50 states included over 84,400 tracts, up from about 73,000 in 2010. In addition to new tracts, boundaries of other tracts were revised through splits and combinations to ensure that they met a number of Census Bureau criteria. These boundary changes prohibit direct comparison of the two vintages.
For this project, the population data from the 2010 and 2020 decennial censuses were combined using a “crosswalk” file from the National Historic Geographic Information System (NHGIS).
Generally, geographic crosswalks provide weights that can be used to compare data for areas with dissimilar boundaries. The NHGIS crosswalks describe how the nation’s 11 million census tabulation blocks in 2010 correspond with 8.1 million published blocks in 2020. Blocks are the smallest geographic area for which the Census Bureau publishes data. The population within the cross-walked blocks can be added together to form tract-level population estimates from both decades.
- View the interactive TNDSC 2010 to 2020 Population Change Map
- Download GIS data (Shapefile 382 MB| File Geodatabase – 181MB)
- Read details of the crosswalk process