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:
- Census enumeration Questions and Forms used in each decade, 1850-2000
- Instructions given to census enumerators, 1850-2000
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 firstname.lastname@example.org 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:
- Biggest Cities Population (1790-1990): We created this layer from a Census Bureau list of the largest-population cities in the USA (up to 100 cities with the largest population in each census decade). We used a variety of sources to find these cities and create a map point for each one (lat-lon coordinates). Note that these cities are points at the center of the city, and not shapes of the boundaries of the city. Finding maps of city boundaries at each decade would be a worthy but significant task.
- Major Railroad Lines (1830-1900): We used a variety of historic railroad maps to create this layer. Note that the latest year represented is 1900. This layer is a work in progress, and we anticipate expanding it significantly in the near future, to include expansion in the 20th century, and the addition of smaller railraod lines.
- Treaties with Lakota, Nakota, Dakota (1850-1894): The shapes in this layer were created by tracing treaty-area boundaries from the maps created for the 1899 publication Indian Land Cessions in the United States by Charles Royce. Images of these maps, and the texts of each treaty represented on them, are available on-line from the Library of Congress collection,"Indian Land Cessions in the United States, 1794-1894." Only the plains territories maps (primarily North and South Dakota) were used for this layer, and only treaties that were listed as having "Sioux" signatories. Categories of treaty types were created by us for the upcoming Black Hills curriculum unit.
More census data and materials on the census
- Social Explorer: An excellent on-line GIS with current and historical census data mapped for the US.
- The main Census 2000 web site: A huge range of materials to explore
- Census in schools: Census-data lesson plans for grades K-12, resources for teachers
- American FactFinder: An easy-to-use resource to easily access census data from 1990 to the present, including query tools, maps, and census bureau reports.
- The Electronic Map Library: a wonderful collection GIS map images (not interactive) created by Dr. William Bowen at UC Northridge, showing 1990 and 2000 census data for many US cities
- Statistical Abstract of the United States: A huge list of reports from census data, 1995-2000, organized by topic and year