Cartographic pet peeve: Who skipped 94101?


For most US cities, I can quickly get a map zoomed right to the central city by going to most any mapping application and typing in a five-digit ZIP code that ends in “01.” That’s because the first three digits of any ZIP code are actually a meta-ZIP, best known by the Census Bureau’s term “ZCTA3.”

Back when ZIPs were assigned, the large post offices that served entire cities or broad rural areas were assigned these three-digit codes, and subareas within them were numbered off in roughly concentric order from the sorting center. ZIP code XYZ01, then, was usually at the then-principal post office, either right downtown or by the railyards where mail was usually offloaded then.

The Post Office assigned these codes in an ascending order following a meandering east-west path across the Lower 48, beginning in New England and ending in the Northwest. Thus, it’s not terribly difficult to remember a city’s ZCTA3 — since they’re systematic, it’s easier than memorizing cities’ telephone area codes (a taxonomy that, while having its own charming history, is increasingly irrelevant).

  • Boston 021
  • New York City 100
  • Pittsburgh 152
  • Philadelphia 191
  • Washington 200
  • Raleigh 276
  • Durham 277
  • Atlanta 303
  • Miami 331
  • Minneapolis 554
  • Chicago 606
  • Denver 802
  • Los Angeles 900
  • Portland 972
  • Seattle 981

BUT there’s one (one!) exception that thwarts my little mnemonic: San Francisco. Its ZCTA3 is 941, but there is no 94101; the lowest-numbered ZIP in town, centered on the main post office by the Civic Center, is 94102. Who stole 94101?!

Postscript via Eric Fischer, proud owner of a pre-ZIP postal map: