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The Decennial Census is required by the United States Constitution to enumerate the population for purposes of congressional representation, that is, for the redistricting process. It is conducted in years ending in zero. From 1940 to 2000, a long form questionnaire was included in the Population Census to collect more detailed data from a sample of the population (about 17%). Those data are now collected via the annual American Community Survey.
Here is a reference guide to the Census providing definitions of terms and geographic areas. define geographic areas. Find reference information such as FIPS and ANSI codes, census tracts, etc.
In addition to the Decennial census the Census Bureau conducts many surveys.
The American Community Survey
The American Community Survey (ACS) began in 2005 as a nationwide survey conducted by the Census Bureau that replaces the “long form” in the decennial census. The ACS collects demographic, housing, social, and economic data on a three million household sample of the United States each year. The Census Bureau releases ACS data on a rolling basis, with three different files covering a one-year, a three-year and a five-year release.
Be careful about comparing ACS data with the decennial census. The Census Bureau provides guidelines:
Quickly look up and download census data. This is also a powerful and comprehensive tool which also includes data from the over 100 censuses and surveys conducted every year by the Census Bureau. This tool also allows the downloading of spatial data, i.e., shapefiles, from the bureau's reference maps.
TIGER provides shapfiles including features such as roads, railroads, rivers, which contain geographic entity codes that can be linked to the Census Bureau’s demographic data. Some of the TIGER/Line Shapefiles are pre-joined with Census data (geodatabases) for easy import into ArcGIS.
The NHGIS provides, free of charge, aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2011. This reource also provides time-series data which link together comparable statistics from multiple censuses in one table. Variables for each time series table are harmonized, meaning they are identical across all years of the time series, making it easier to compare data across time.
ESRI provides an array of easily downloadable map layers illustrating Census data. You can also import them instantly into ArcGIS Online, edit and share them on the internet (after creating a free account). See below for making maps online.