The IPUMS National Historical Geographic Information System (NHGIS) provides free online access to summary statistics and GIS files for U.S. censuses and other nationwide surveys from 1790 through the present. NHGIS boundary files are derived primarily from the U.S. Census Bureau's TIGER/Line files with numerous additions to represent historical (1790-1980) boundaries that do not appear in TIGER/Line files. For more recent boundary files (1990 or later), NHGIS typically makes only a few key changes to the TIGER/Line source: (1) we merge files that are available only for individual states or counties to produce new nationwide or statewide files, (2) we project the data into Esri's USA Contiguous Albers Equal Area Conic Projected Coordinate System, (3) we add a GISJOIN attribute field, which supplies standard identifiers that correspond to the GISJOIN identifiers in NHGIS data tables, (4) we rename files to use the NHGIS naming style and geographic-level codes, (5) we add NHGIS-specific metadata, and (6) most substantially, we erase coastal water areas to produce polygons that terminate at the coasts and Great Lakes shores.

NHGIS derived this shapefile from the U.S. Census Bureau's 2020 TIGER/Line Shapefiles. The TIGER/Line technical documentation is available through the NHGIS website at https://www.nhgis.org/gis-files#tiger-doc.