This post is part of our practical cartography series.
Most American geographers will note that - as much as we'd like it to be otherwise - ZIP Codes are not polygons. Rather, they're constantly-changing lines used by the USPS to coordinate delivery in an efficient network. Many of us polygon-happy mappers use ZIP Code Tabulation Areas (ZCTAs) instead; these are provided by the US Census as a reasonable open data alternative to ZIPs. They're particularly nice for thematic mapping (though their shortcomings have also been well-documented):
But why use ZCTAs if they can never be reconciled with their ground-truth ZIP cousins?
Because the difference is small.
Faraday has address and location records for every household in the country, and it was straightforward to check for disagreement between the ZIP Code of each physical address and the ZCTA polygon that contains it.
The national error rate of ZCTAs is 1.4%. That might be too high for some use cases, but perfectly acceptable for others. There's some regional variation, too: you're usually safe to use ZCTAs in Hawaii and Maine, but might want to exercise caution in Oregon and Utah.