What is the source of NHGIS data?
The original source of most of the aggregate data, with a few exceptions, is the US Census. For censuses since 1970, the NHGIS obtained digital data directly from the Census Bureau. For earlier censuses, NHGIS data are generally derived from secondary sources: separate projects which have, over the course of several decades, undertaken the arduous conversion of pre-computer-age historical data from print media to a digital, machine-readable form. Most of this work was completed by Michael Haines, Donald Bogue, Andrew Beveridge and their respective research teams. Documentation for most sources is available on the Tabular Data Documentationpage.
The GIS boundary files are based on the TIGER/Line data that the US Census Bureau creates, which NHGIS staff edited to produce all pre-1990 boundaries. The primary guide for historical census tract boundaries was original census maps and, for early county boundaries, it was the book, Map Guide to the U.S. Federal Censuses 1790-1920, by William Thorndale and William Dollarhide (Genealogical Publishing Co., Baltimore, MD, c. 1987).