A lot of digital maps use zip codes as a binning feature. Election maps, property value maps, pollution maps. But while zip codes are convenient and familiar there’s a much better set of polygons for mapping most US data: census tracts. Zip codes are pretty unusual polygons(*), drawn mostly to make the process of mail sorting and delivery simpler. They are quite arbitrary. Census tracts are lovingly and carefully drawn to respect demographic and political reality.
I have a great example of the difference: River Oaks in Houston, one of the wealthiest neighborhoods in America. It’s the place where Saudi princes buy a 10,000 square foot house and glass in the backyard so they have air conditioning. Also plenty of local Houston money, mostly old money.
But you’d never know it looking at zip code averages. Because River Oaks shares 77019 with a bunch of people who live east of Shepherd, in a significantly denser, less wealthy, more diverse neighborhood. It also includes big chunks of the Fourth Ward which used to be quite a poor neighborhood and was recently redeveloped. By contrast River Oaks is almost perfectly described by census tracts 4112 and 4114. (Not exactly; the chunk in the SW of 4114 is not technically River Oaks, nor is it in 77019, but demographically it’s quite similar to the rest of 4114.)
Zip code 77019
River Oaks
Census Tracts
Census tracts are part of a hierarchy of census polygons. Census tracts aim to have about 4000 people in them, (zip codes average 8000 people, but vary greatly). Block groups are smaller and have ~1500 people each. Census blocks are the smallest division, about 40 blocks per group, but unlike census tracts they are highly variable (in terms of statistical demographics). There are 11M census blocks in the US, although nearly half of them are uninhabited.
Census tracts are frequently named by 4 digit numbers like 4112 or 4114, with a county name or something else to disambiguate them. But some tracts have numbers with decimals or other complicating factors, like 32 or 204.01. (I believe canonically census tracts are named with 6 digit numbers, with the decimal and leading zeros frequently omitted).
FIPS codes are the larger naming scheme; the full name of 4112 is sometimes given as 48201411200. 48 is Texas, 201 is Harris County, 4112 the census tract, and the 00 is the block group. Census blocks get their own 4 digit number at the end, making for a full 15 digit code to name the most precise polygon.
(*) technically speaking zip codes are actually just lists of points. The polygons are properly called zip code tabulation areas and there are some important differences.
