Each of the data sets is available at any of the following 5 levels of geography:
- States
- Counties
- Tracts
- Block Groups
- Zip Codes
Zip code definitions come from at least two different federal agencies and they
don't always line up perfectly. The data in this product defines zip codes
using the US Postal Service (USPS) definitions. But the geographic boundaries
are derived from the Census Bureau's (USCB) Tiger files and are officially Zip
Code Tabulation Areas (ZCTA). Most of the time these two are relatively in
accord. But there are some differences. For example the USPS has zip codes
for PO Boxes. The USCB does not - no one "lives there" though many people may
receive mail there and it is an official mailing address. Also the USCB seeks
to cover the entire country so for example it will assign NY City's Central Park
or Yosemite to a zip code. The USPS may not include these areas because in fact
they are not official mailing addresses.
