About the Data

The historical county maps

US counties have changed a lot over time, and it is important to view the census data in the accurate shape of the county (or state) as it existed at that time. Our GIS maps use the most recent county shape files created by the National Historical GIS Project currently in beta release. The shape files are created by tracing historic maps, and consulting documents describing the boundaries of each county in each census year.

NHGIS is a partner in this project, and we will update the site with new files as they become available. For improved loading of the web site we are currently pursuing a "thinned" set of shape files, in which detailed contours of some counties have been simplified.

For more detailed information on changing county shapes over time, John Long's Atlas of Historical County Boundaries at the Newberry Library is the definitive source. Dr. Long is an advisor to our project.

The historical census data

The data used in the GIS are provided by the Inter-University Consortium for Political and Social Research (ICPSR). The source file for most of the data is: HISTORICAL, DEMOGRAPHIC, ECONOMIC, AND SOCIAL DATA: THE UNITED STATES, 1790-1970 [Computer file]. Ann Arbor, MI: ICPSR [producer and distributor].

The larger set of data from this source can be easily browsed on-line on the University of Virginia's Geostat Center, a wonderful web site where students can do further investigations. More census data can be found at the University of Minnesota's Integrated Public-Use Microdata Series (IPUMS). There are many more data variables on these sites that are not currently used in our GIS.

The IPUMS web site also has copies of the census materials used each decade, which are a great resource for helping students understand the census data - where it comes from, what each variable means, and why it was collected:

Where do the data come from, and what do they mean?

The source book describing the census data variables we used is the ICPSR's Historic, Demographic, Economic and Social Data: The United States, 1790-1970, available for download from ICPSR. This PDF document is large (22.6 MB) and may be difficult to download at slow connection speeds.

A booklet describing the census data collected each year is Measuring America: The Decennial Censuses from 1790 to 2000, available for download from the US census web site. This PDF document is large (15.3 MB) and may be difficult to download at slow connection speeds. Please email comments@census.gov if you have problems with this link.

The Geostat Center's data documentation page is very thorough in describing the source data used in both their web site and ours. It also has useful links to further census data resources.

Original layers created for this project

For some of the projects on this site we created original GIS layers from a variety of sources:

More census data and materials on the census

We the People