Census Workshop

I think the important thing to highlight here is that:

• some geographies are nested (i.e., census blocks on up through the center line),
• while some are not (i.e., core-based statistical areas aka metro areas),
• and some are not that you might expect would be nested (i.e., ZIP Code Tabulation Areas aka ZCTAs aka the Census’s attempt at zip codes).

What are these things anyways?

• Blocks: statistical areas bounded by visible features.  Generally, they are small in area, much like a block in a city bounded by streets.  But in rural areas they can be very large, irregular, and bounded by a variety of features, such as streams.
• Block Groups: A block group is a cluster of blocks that are nested within a census tract.  They contain between 600 to 3,000 people.
• Census Tracts:  A census tract is a cluster of block groups nested within a county.  They contain between 1,200 to 8,000 people, with an optimum size of 4,000 people.  Census tracts are delineated with the intention of being maintained over a long period of time, so that comparisons between censuses can be made.  However, census tracts can and do split or merge based on population changes.
• A complete glossary of Census geography terms can be found at the Geography Program.

Don't forget that Social Explorer makes it very easy to find geographies with an Address Search or by asking for multiple geographies.  If you want to find information by an address, start with the Mapping feature.  If you want to pull multiple geographies, start with the Table feature. 

Each geography comes with a geographic identifier called a FIPS code.  This code helps track geographies, which is useful when you need to join different Census tables or other non-Census surveys, or map Census data with a GIS.  

FIPS codes for smaller geographic entities are usually unique within larger geographic entities.  For example, state codes are unique within the nation, and county codes are unique within the state.  Let's take a look at the first 3 county FIPS codes in Virginia: 

The first two digits - 51 - represent the Virginia state FIPS code, and the last 3 digits represent the counties within Virginia. 

In Virginia, all municipalities incorporated as cities are "independent cities."  While almost all American cities are embedded within a single, or possibly multiple, counties, Virginia cities are not.  For example, the City of Charlottesville exists as an independent city, with the donut of Albemarle County surrounding it.  Charlottesville is not part of Albemarle.

The census handles Virginia independent cities BOTH as cities AND as though they were counties.  You can find independent city data in county-level tables, which is convenient when you are pulling data for several Virginia counties. 

The curious might enjoy this Wikipedia entry on the history of independent cities in Virginia:

If you are interested in the differences between urban and rural areas, do yourself a favor and read up on the Census's definition of those areas.  

The Census Bureau identifies two types of urban areas:

• Urbanized Areas (UAs) of 50,000 or more people;
• Urban Clusters (UCs) of at least 2,500 and less than 50,000 people.

“Rural” encompasses all population, housing, and territory not included within an urban area.

Just for fun, here is a link to the 2010 Census - Charlottesville Urbanized Area map.  Notice that it is larger than the Charlottesville city limits.