For an overview of the geographic units and years covered by NHGIS boundary files, see Data Availability.
NHGIS boundary files are shapefiles, a standard spatial data file format. The shapefile format was originally defined for use in Esri GIS applications, but the format has become an industry standard, and many GIS and mapping tools are able to read and write shapefile data.
NHGIS generally identifies boundary files by the survey year in which the boundaries were used for tabulations, which may be different than the vintage of the boundaries. For example:
- The 2012 boundary file for Core Bases Statistical Areas (CBSAs) follows the official 2009 CBSA delineations, which are the delineations used in 2012 American Community Survey (ACS) tables.
- The 2010 and 2011 boundary files for Public Use Microdata Sample Areas (PUMAs) identify 2000 PUMAs, which are the PUMAs used in 2010 and 2011 ACS tables.
This Census Bureau page identifies the vintages of geographic areas for each ACS survey year since 2009.
Note on 2009 census tracts and block groups:
To find the NHGIS boundary files for block groups and census tracts derived from 2009 TIGER/Line files, users should filter on the year 2000 in the Data Finder. NHGIS identifies these boundaries with 2000, not 2009, because they do not completely correspond to the units used in 2009 ACS tables. Most of the block group and census tract tables from the 2009 5-Year ACS Summary File correspond to the Census 2000 definitions, but according to ACS documentation, "in 19 counties from 8 different states, many of the census tracts and block groups used to tabulate and present the 2005-2009 ACS 5-year estimates are either those submitted to the Census Bureau for the 2010 Census, or a preliminary version of 2010 Census definitions." More information on these discrepancies, including a listing of affected counties, is available here.
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. NHGIS has also erased coastal water areas in all years to produce polygons that terminate at the U.S. coasts and Great Lakes shores (unlike TIGER/Line polygons).
2000 and earlier boundaries
For 1990 and 2000 areas, NHGIS modified the 2000 TIGER/Line definitions only by erasing coastal water areas. Because the 2000 TIGER/Line files contain no identifiers for census areas from 1980 and earlier, NHGIS researchers obtained boundary definitions for those years by consulting other sources, including 1992 TIGER/Line data for 1980 census tracts; maps from printed census reports for 1910-1980 census tracts and other small areas; and the Map Guide to the U.S. Federal Censuses, 1790-1920, by William Thorndale and William Dollarhide (Genealogical Publishing Co., Baltimore, MD, 1987), for counties and states back to 1790. Where the historical boundaries follow 2000 TIGER/Line features, the original NHGIS boundary files re-use those TIGER/Line features. Elsewhere, NHGIS researchers digitized new boundaries. NHGIS boundary files based on these files are identified as "2000 TIGER/Line +" in the Basis column in the Select Data grid of the Data Finder.
1980 place and county subdivision boundaries
1980 boundaries for places and county subdivisions are derived from the U.S. Census Bureau's 1992 TIGER/Line files. NHGIS modified the TIGER/Line definitions only by erasing coastal water areas using the 2000 TIGER/Line coastal water definitions. NHGIS boundary files based on these files have a "1992 TIGER/Line +" Basis in the Data Finder.
2000 boundaries based on 2010 TIGER/Line files
NHGIS also provides 2000 boundaries derived from the 2010 TIGER/Line files, which have a "2010 TIGER/Line +" Basis in the Data Finder. For these, NHGIS modified the TIGER/Line definitions only by erasing 2010 coastal water areas. The 2000 boundaries derived from 2010 TIGER/Line will better align with 2010 and newer GIS boundary files.
2009 and later boundaries
NHGIS generally derived 2009 and later boundaries from the corresponding TIGER/Line release, as indicated by the Basis in the Data Finder. In each case, NHGIS modified the TIGER/Line definitions only by erasing coastal water areas. For the 2009 boundary files, NHGIS used 2010 TIGER/Line coastlines to erase coastal water areas.
Because the Census Bureau made major accuracy improvements to TIGER/Line features between the 2000 and 2008 TIGER/Line releases, the original NHGIS shapefiles based on 2000 TIGER/Line features are not comparable with newer TIGER/Line data. We therefore generated new 2008-based boundary files by systematically realigning the boundaries for tracts and counties to fit with 2008 TIGER/Line features, a process referred to as conflation. These conflated NHGIS boundary files are identified as "2008 TIGER/Line +" in the Basis column found in the Select Data grid.
The Census Bureau made additional improvements to TIGER/Line features after 2008, so the 2008 TIGER/Line-based files are not consistently comparable with 2010 and later TIGER/Line files. In general, most 2008-based boundaries align better than 2000-based boundaries with 2010 and later TIGER/Line files, but the 2008-based boundaries also include occasional gross inaccuracies.
For users who have no need to compare historical boundaries with boundaries from 2010 or later, we recommend using the original 2000-based NHGIS boundary files.
For users who do wish to compare or overlay historical boundaries with boundaries from 2010 or later, we recommend downloading and examining both the 2000- and 2008-based versions of historical boundaries in order to determine which is more suitable for your study area and analysis.
For users who wish to overlay 2000 boundaries with boundaries from 2010 or later, we recommend using the 2000 boundaries derived from 2010 TIGER/Line.