The U.S. Census Bureau keeps track of geographic boundaries for tabulation purposes. In addition to political boundaries such as states and counties, the Census Bureau also creates smaller levels of geography so that data can be tabulated to smaller units. The smallest units are census blocks, followed by block groups and census tracts. The diagram to the right illustrates the hierarchical relationship of these smaller census summary levels.
Below are more detailed descriptions for all of the summary levels for which the Census Bureau tabulates data. All of these census summary levels are available for Caliper mapping software products and include 2010 Census data and most include the latest American Community Survey (ACS) data. Caliper also offers 1990 Census data and 2000 Census data.