The NHGIS is a project at the University of Minnesota designed to collect GIS data for the United States at the smallest possible Geographic level. Currently the geographic boundary files include states and counties from 1790 to the present and census tracts from 1910 to the present.

The Social Explorer produces maps and reports of US Census Tract level data from 1940-2000. Using high quality maps and data, they make available the full contents of the tract data for the United States for the full period. The data can also be exported as excel spreadsheets for easy reuse.