I think your general understanding is correct, but I’ll do my best to explain the relevant concepts to make sure…
First, I’ll explain the “basis” of GIS files. Then I’ll explain how the GIS files correspond to the two types of time series tables.
The “basis” of the GIS files doesn’t describe the year when the boundaries were in use. It describes which release of the Census Bureau’s TIGER/Line files NHGIS has used as the geometric basis for the boundaries.
The TIGER/Line files include geometric definitions of many features, e.g., county lines, streets, rivers, coastlines, etc. The Census Bureau regularly releases new versions of TIGER/Line files, and each new version includes various accuracy improvements in the representations of features. As such, the geometry for a single feature (a street, a river, a county line) often differs from one TIGER/Line release to another as the representations are improved.
When two NHGIS GIS files differ only in their basis, that indicates that they provide different geometric representations for the same features. For example, NHGIS provides four shapefiles representing the boundaries of 2000 census tracts, each with a different TIGER/Line basis:
The “YEAR” column indicates that all four of these shapefiles represent 2000 census tracts. You could use any of them to map and analyze data for 2000 census tracts.
In the file with a 2000 TIGER/Line+ basis, the tract boundary geometry corresponds to features (streets, rivers, county lines, etc.) as they were represented in the 2000 TIGER/Line file. The file with a 2008 TIGER/Line+ basis also represents 2000 census tracts, but the boundary geometry corresponds to features (streets, rivers, county lines, etc.) as represented in the 2008 TIGER/Line file.
Now, about time series tables…
You can learn more about the two types of geographic integration NHGIS uses in time series tables through links in the NHGIS Data Finder (click on the “Nominal” or “Standardized to 2010” links) or in the Geographic Integration section of the Time Series Tables.
Importantly, the nominally integrated tables don’t report data for static geographic units across time:
Nominally integrated tables link geographic units across time according to their names and codes, disregarding any changes in unit boundaries. The identified geographic units match those from each census source, so the spatial definitions and total number of units may vary from one time to another (e.g., a city may annex land, a tract may be split in two, a new county may be created, etc.). The tables include data for a particular geographic unit only at times when the unit’s name or code was in use, resulting in truncated time series for some areas.
This means that 1970 data in a nominally integrated table correspond to 1970 census boundaries, and 2020 data correspond to 2020 census boundaries, etc.
To map the 1970 data in a nominally integrated table, you need to get a GIS file for 1970 geographic units (1970 counties or 1970 tracts, etc.). You could use the version with a 2000 TIGER/Line+ basis or a 2008 TIGER/Line+ basis. Regardless of the basis you choose, to map the 2000 data in a nominally integrated table, you’d still need to get a different GIS file, one that represents 2000 census geographic units.
Only the time series tables that are “Standardized to 2010” provide estimates for static geographic units across time. To map data in these tables, you need to get a GIS file that represents 2010 census units. NHGIS has GIS files for 2010 census units with either a 2010 TIGER/Line+ basis or a 2020 TIGER/Line+ basis. You could choose either of these.
