I am doing home for my GIS class where we had to download census data from our county and to define what geoid10 and its numbers mean. I cannot find the definition anywhere.
GEOID is the field used to join TIGER/Line geographic data to the demographic data in various American Community Survey products and in the Decennial Census. It is slightly confused by the fact that this field is called GEOID10 only in the TIGER/Line 2010 products (in fact, almost all of the field names in TIGER/Line 2010 end in 10), and by the fact that the actual code is different in the geographic and demographic products and has to be manipulated for the join to work.
This is discussed in several place, but most clearly in various ACS Technical Documents. See for example section 2.6 "How to Join the ACS Summary File to the TIGER/Line Shapefiles" of the 2006-2010 ACS 5-Year Summary File Technical Documentation.
In the ACS, GEOID begins as follows:
- 3 digits for the summary level
- 040 = state
- 050 = county
- 2 characters for the component
- There are a large number of geographic components, but mostly what you need to know is 00 means the entire population, while anything else means only part of the population, e.g. the urban part, the rural part, etc.
- The letters US
After those 7 characters which are not present in the TIGER/Line products, the ACS GEOID and TIGER/Line GEOID fields will match. The code will be a concatenation of the code of every geographic entity in the hierarchy of that record. For a tract, this may be two digits for the state FIPS, three digits for the county FIPS, and six digits for the tract code—or it may not, because another version of the census tract (summary level 080) follows the hierarchy State-County-County Subdivision-Place/Remainder-Census Tract.