Geography is at the center of taking a census. We do not just count people; we count people where they live. Geography is important because it is the basis for taking a census and for tabulating census data. The Census Bureau also maintains unique geographic areas that other local, state and federal agencies use.

Understanding geographic relationships is key to understanding how to properly use Census Bureau data. This is the first in a series of posts that will shed some light on how these different entities relate to one another. Part one focuses on geographic relationships that exist below the national level, such as ZIP Code tabulation areas and school districts.

The Standard Hierarchy of Census Geographic Entities [PDF] displays the relationships between legal, administrative and statistical boundaries maintained by the Census Bureau. It depicts relationships with a line and shows where relationships do not exist by displaying entities on different line tracks. In short, it shows how different geographic areas may, or may not, be related.

Here are some examples to explain how some of the relationships on the hierarchy work.

• ZIP Code Tabulation Areas (ZCTAs) are based on the U.S. Postal Services ZIP Codes and must fall within the national boundary only. Many data users think that ZCTAs must stay within the state boundary, but in a few cases, ZCTAs can cross into bordering states.
• School districts must fall within each state. States are responsible for updating their boundaries, and districts may cross county and place boundaries.
• County subdivisions, as the name suggests, must fall within the county. Many county subdivision names repeat throughout the nation and throughout the same state, so it is important you know which county you are working in. For example, in 2010, Beaver was used as the name of 45 different county subdivisions.
• Places stay within state boundaries. Many place names repeat throughout the country (e.g. Kansas City, Kan.  vs. Kansas City, Mo. ), but each is unique, with different mayors, schools and services. In a few cases, place names can even repeat in the same state (e.g. Aaronsburg CDP in Pennsylvania occurs twice).
• Urban areas fall within the nation because they do not have to conform to place, county or even state boundaries.
• Core Based Statistical Areas (CBSAs) (metropolitan/micropolitan statistical areas) fall within the nation and often cross state lines.
• Census tracts must stay within a county and therefore a state. They do not necessarily coincide within any other geography. For example, although some census tracts follow place boundaries, there is no rule that says they must stay within a place.
• Block groups must stay within each census tract, so they must also stay within a county and state.
• Blocks fall within everything! They are the building blocks for all other geographies and therefore nest within all other geographies. Their four-digit codes correspond to the block group that they fall within. So, if you want data for a specific block, you must also know the block group, census tract, county and state it exists in.

The hierarchy provides a quick and easy way for data users to see how the different geographic entities at the Census Bureau relate to one another. It is important to understand the hierarchy to get to the correct data. The next Understanding Geographic Relationships post will discuss the hierarchy of geographic entities related to American Indian Areas.

It is the 25 year anniversary of the TIGER (Topologically Integrated Geographic Encoding and Referencing) database. Stay tuned for more on this important milestone in geographic history.