About ZIPs, ZCTAs, and their relationship to other local boundaries
While most of us think of them as geographic areas, ZIP (“Zone Improvement Plan”) codes are really just collections of addresses created by the U.S. Postal Service to make delivery of mail more efficient. Each address’s ZIP code ties that address to a particular post office or delivery station. ZCTAs (ZIP Code Tabulation Areas), on the other hand, are geographic area boundaries created by the U.S. Census Bureau to roughly represent the geometric “shape” of the related physical addresses on a map. Usually, the ZCTA is the same as the zip code for an area, but not always. And not all zip codes are captured by a ZCTA, particularly those where the zip code represents just one mailing address, as is often the case for large businesses. (Read more about ZIP codes and ZCTAs on the U.S. Census Bureau’s website.)
ZCTAs have no relationship at all to Census divisions (tracts, block groups) or to jurisdictional divisions like City Council Districts, County Commissioner Precincts, school districts, election precincts, or state or national legislative districts. That means that ZCTA boundaries will never match up with any of those other geopolitical boundaries. But ZCTAs are still useful to get a sense of conditions in a school district or City Council district. The Filter view in The Viz-a-lyzer allows the user to do just that, showing only the ZCTAs in the area of interest and hiding the others.
To decide which ZCTAs to tie to a given jurisdictional division (e.g., a City Council District) in The Viz-a-lyzer, CI:Now overlaid and visually compared the ZCTA boundaries with the geopolitical boundary in question. If the ZCTA and the jurisdictional division (say, District 5) overlap to any significant degree, the ZCTA shows up the D5 Filter view. That’s the case even if the ZCTA is partly or wholly in another jurisdiction, like another city or a federal military base. If the ZCTA and the jurisdictional division have only a tiny amount of overlap or no overlap at all, the ZCTA is hidden. The intention is to show a “common sense” set of ZCTAs for a given jurisdictional division, and in many cases, the decision of whether to include a ZCTA is a judgment call.
Please note that the Filter just provides a view of data for individual ZCTAs. The Viz-a-lyzer doesn’t actually calculate any indicators for a group of ZCTAs or for any other geographic level.