IPUMS NHGIS place point GIS files depict the locations of incorporated, unincorporated, and census-designated places for the entire U.S. from 1900 to 2015.
Overview
NHGIS place point GIS files depict the locations of incorporated, unincorporated, and census-designated places identified in the U.S. Census Bureau's published datasets and volumes. Place point files are available for decennial census years from 1900 through 2010 and for each individual year from 2009 through 2015.
The place points are derived from several sources, including the U.S. Board on Geographic Names' Geographic Names Information System (GNIS), place boundary polygons from the Census Bureau's 1992, 2000, and 2009-2015 TIGER/Line files, and scanned maps and lists of incorporated and unincorporated places from 1930-1970 U.S. Census printed publications.
Wherever possible, NHGIS place points are based on GNIS coordinates, which generally gives each place's point a fixed position across time, located within the historical, functional center of the place (e.g., the central business district of a city). TIGER/Line files provide only place boundaries and the geographic centers of place extents, features which often change over time because of place definition changes, annexations, or improvements in TIGER/Line representations, and the geographic centers may be far removed from functional place centers. For historical census places with no corresponding GNIS point, NHGIS digitized the place point from scanned, georeferenced census maps.
For example, the city of Denver, Colorado, has continually annexed territory throughout its history, including an addition of 53 square miles in the 1980s for a new airport, which shifted its geographic center several miles to the east. In NHGIS point files, however, the Denver point has a single, unchanging GNIS-based position located at the Colorado State Capitol in the historical, functional core of Denver.
Currently, NHGIS provides population and housing data tables for places from 1970 to 2015. Users may join the data tables to the place point files using the GISJOIN attribute. The files contain another unique identifier, the NHGISPLACE code, that is consistent across time, so it can be used to join point files together and to identify the years for which each place appears in census data, regardless of place name or census code changes.
Production Details
Creating the place point GIS files involved four processes. This section describes each process.
1970-2015 Places
Working from NHGIS place boundary files, which are available for years from 1980 to 2015 and derived primarily from TIGER/Line files, we converted each place polygon to a point located at its geographic centroid, merged all years' points into a single file, and removed records with duplicate names and GISJOINs. This step yielded a dataset with a single point for each unique place for the entire period covered by NHGIS boundary files.
We then joined the place points to an NHGIS nominally integrated time series table (TST) for places, using the NHGISCODE field in the TST and the GISJOIN field in the point file. The TST links place data together from the 1970-2010 decennial censuses and 2008-2012 American Community Survey, providing a list of years for which each place exists in the census data. This join generated two useful pieces of information. First, place points with a match in the TST could be extended back to 1970. Second, TST records without a match in the place points represented features to be added to the point dataset. All TST records without a match existed only in 1970, and they had no match because there were no GIS data for 1970.
Next, we updated the geographic coordinates of the NHGIS place points to match GNIS coordinates. We first converted GNIS populated place coordinates to a GIS file and then joined the TIGER/Line-and-TST-based place points to the GNIS file using a combination of place, county, and state names. For matched records, we changed the coordinates of the NHGIS place points to the GNIS locations. For unmatched records with an NHGIS point, the NHGIS coordinate was retained. Finally, for unmatched records without an NHGIS point, we used a combination of GNIS points and scanned, georeferenced 1970 Census Bureau maps to locate place points.
At the end of this process, we had created a single dataset containing points for all incorporated, unincorporated, and census-designated places in the 1970-2010 decennial censuses and the 2009-2015 American Community Survey datasets.
1930-1960 Places
Our primary source material for 1930-1960 places were lists of incorporated and unincorporated places and scanned, georeferenced maps from Census Bureau publications. Research staff worked on one state at a time, backwards from 1960 to 1930.
For each state, we overlaid our existing place points with the scanned, georeferenced maps and the GNIS points. Working from the list of incorporated and unincorporated places, researchers would look up each place in the place point dataset. If it existed, the researcher would flag the point record accordingly. If it did not exist, the researcher would create a new point, either by snapping to a GNIS feature or by digitizing the new point at the approximate geographic center of the place on the scanned map.
1900-1920 Places
Our primary source material for 1900-1920 places consisted only of lists of incorporated and unincorporated places from Census Bureau publications. There were no maps depicting the locations of places for these years.
For each state, research staff overlaid our existing place points with the 1930 scanned, georeferenced maps and the GNIS points. As was done for 1930-1960 places, a researcher would look up each place from the list of incorporated and unincorporated places in the place point dataset. If it existed, the researchers would flag the point record accordingly, working on the assumption that the 1930 (or later) point location was a suitable representation of the pre-1930 place center. If our place file did not contain a corresponding point, the researcher would research the place. First, if the Census Bureau's published lists for place name changes indicated the historical place matched a place in a later census, we re-used that place's coordinates. Second, if the GNIS data contained a matching point, we used the GNIS coordinates. Finally, we would research the place name on a variety of web resources (e.g., local historical societies or genealogical sites) and digitize a location based on the research.
Quality Assurance and Quality Control
For each state's place points, research staff executed a three-part quality-checking process.
First, we overlaid the place points for 1930-1970 with the corresponding scanned map for each year and carefully panned across the map, looking for any discrepancies between place points and places on the scanned map.
Second, we compared the number of place points for each state in each year to the number of place records in the published census volumes.
Finally, we identified any points falling outside the correct state due to being snapped to incorrect GNIS points (e.g., GNIS points with the same place name but in a different state).
In each case, we investigated and resolved any discrepancies.
Place Identifiers in the GIS Files
NHGIS place point files use two types of codes to uniquely identify places.
- GISJOIN identifiers match the unique identifiers in NHGIS data tables from the same year as the place point file. GISJOIN codes are not provided in point files for 1960 or earlier years because NHGIS does not currently include any place data tables for those years. A place GISJOIN concatenates these codes:
| Component | Notes |
|---|---|
| "G" prefix | This prevents applications from automatically reading the identifier as a number and, in effect, dropping important leading zeros. |
| State NHGIS code | 3 digits (FIPS + "0"). NHGIS adds a zero to state FIPS codes to differentiate current states from historical territories. |
| Place FIPS or census code | 5 digits for 1990 and later years. 4 digits for 1980 and 1970. |
- NHGISPLACE identifiers are provided in all place point files. They are consistent for each place across time, so they may be used to identify the same place in different years, regardless of changes in place names or census codes. The NHGISPLACE concatenates these codes:
| Component | Notes |
|---|---|
| "G" prefix | This prevents applications from automatically reading the identifier as a number and, in effect, dropping important leading zeros. |
| State NHGIS code | 3 digits (FIPS + "0"). NHGIS adds a zero to state FIPS codes to differentiate current states from historical territories. |
| Place NHGIS code | 6 digits ending with a "0" or "5". The first 5 digits generally correspond to the place FIPS code if the place is identified in recent (1990 or later) census data. NHGIS place codes ending in "5" identify historical places that do not appear in recent census data. Codes ending in "0" identify places that do appear in recent census data. |
Download
Users may download place point GIS files through the NHGIS Data Finder. When a data filter is selected, the available place point files appear under the "GIS FILES" tab with a geographic level of "Place (Points)". Files with a geographic level identified only as "Place" contain boundary polygons, not points.
Citation and Use
Use of the NHGIS place points is subject to the same conditions as for all NHGIS data. See NHGIS Citation and Use.