Understanding Geographic Identifiers (GEOIDs) (2024)

The Census Bureau and other state and federal agencies are responsible for assigning geographic identifiers, or GEOIDs, to geographic entities to facilitate the organization, presentation, and exchange of geographic and statistical data. GEOIDs are numeric codes that uniquely identify all administrative/legal and statistical geographic areas for which the Census Bureau tabulates data. From Alaska, the largest state, to the smallest census block in New York City, every geographic area has a unique GEOID. Some of the most common administrative/legal and statistical geographic entities with unique GEOIDs include states, counties, congressional districts, core based statistical areas (metropolitan and micropolitan areas), census tracts, block groups and census blocks.

GEOIDs are very important for understanding and interpreting geographic and demographic data and their relationship to one another. Data users rely on GEOIDs to join the appropriate demographic data from censuses and surveys, such as the American Community Survey (ACS), to various levels of geography for data analysis, interpretation and mapping. Without a common identifier among geographic and demographic datasets, data users would have a difficult time pairing the appropriate demographic data with the appropriate geographic data, thus considerably increasing data processing times and the likelihood of data inaccuracy.

The American National Standards Institute (ANSI), US Census Bureau, US Department of Education, US Geological Survey (USGS) and individual states all maintain GEOIDs contained in census products. The ANSI, in particular, is responsible for maintaining Federal Information Processing Series (FIPS) codes and Geographic Names Information System (GNIS) codes. A wide audience uses FIPS codes and GNIS codes across many private and public datasets to uniquely identify geographic features.

The Census Bureau has published FIPS codes in census products for more than 30 years. FIPS codes are assigned alphabetically by geographic name for states, counties, core based statistical areas, places, county subdivisions, consolidated cities and all types of American Indian, Alaska Native, and Native Hawaiian (AIANNH) areas. Lists of geographic FIPS codes in census products can be found on the ANSI/FIPS Codes webpage.

FIPS codes for smaller geographic entities are usually unique within larger geographic entities. For example, FIPS state codes are unique within nation and FIPS county codes are unique within state. Since counties nest within states, a full county FIPS code identifies both the state and the nesting county. For example, there are 49 counties in the 50 states ending in the digits “001”. To make these county FIPS codes unique, the state FIPS codes are added to the front of each county (01001, 02001, 04001, etc), where the first two digits refer to the state the county is in and the last three digits refer specifically to the county.

Geographic features in the GNIS do not have nesting relationships, as they do in the FIPS database. Instead, GNIS codes are assigned sequentially, in chronological order, based on date of entry in the database. Many cultural and physical geographic features are codified in the GNIS, including airports, beaches, cemeteries, churches, hospitals, islands, lakes, populated places, post offices, rivers, schools, streams and swamps. The table below illustrates how FIPS codes and GNIS codes are codified and how the databases differ from one another.

The Census Bureau creates and maintains geographic codes for many statistical geographic areas that are not covered by FIPS codes and GNIS codes. These geographic areas include census divisions, census regions, census tracts, block groups, census blocks and urban areas. The full GEOID for many levels of geography combines both the FIPS codes and Census Bureau codes. For example, census tracts, block groups and census blocks nest within state and county; therefore, the GEOIDs for each of these geographic areas contains both the state and county FIPS codes, in which they nest.

The US Department of Education is responsible for issuing and maintaining geographic codes for elementary, secondary and unified school districts throughout the nation. Also, individual states use state-defined standards to establish geographic codes for voting districts and state legislative upper (senate) and lower (house) districts. Voting districts nest within counties and consequently, voting district GEOIDs contain both state and county FIPS codes. Similarly, state FIPS codes comprise a portion of school district and state legislative district GEOIDs, as they share a nesting relationship within states.

The Standard Hierarchy of Census Geographic Entities diagram illustrates the hierarchal relationship of different geographic areas to one another. This diagram is a great tool for understanding how GEOIDs are concatenated for geographic areas that nest within other geographic areas. The table below shows the GEOID structure in TIGER/Line Shapefiles for some of the most common legal and statistical geographies, as well as example GEOIDs for different geographic areas.

Data downloaded from data.census.gov containstwo fields of GEOID information, “GEO.id” and “Name”. The table below shows thefields as they appear in a file downloaded from the data.census.gov searchresults page. The first row in the table contains data field names. The secondrow contains table descriptions. Rows 3-5 contain GEOID information for threecounties in the state of Delaware.

The “GEO.ID” field contains 14-digit codes that identify the summary level of data, the geographic component of the data and FIPS codes that uniquely identify the data. For example, the 14-digit “GEO.ID” for Harris County, TX is “0500000US48201” where “050” represents the summary level of the data, “0000” represents the 2-digit geographic variant and the 2-digit geographic component, “US” represents the United States, “48” represents the state of Texas and “201” represents Harris County.

For a step-by-step guide to joining data downloaded from data.census.gov to TIGER/Line Shapefiles, please see.