Developer Notes

?get_zcta_crosswalk

Start by looking at the contents of the function ?get_zcta_crosswalk:

get_zcta_crosswalk = function() {

  url = "https://www2.census.gov/geo/docs/maps-data/data/rel2020/zcta520/tab20_zcta520_county20_natl.txt"
  zcta_crosswalk = read_delim(file = url, delim = "|")

  # Select and rename columns
  zcta_crosswalk = zcta_crosswalk |>
    rename(zcta        = .data$GEOID_ZCTA5_20,
           county_fips = .data$GEOID_COUNTY_20,
           county_name = .data$NAMELSAD_COUNTY_20) |>
    select(.data$zcta, .data$county_fips, .data$county_name)

  # 1. The county FIPS is always 5 characters. And the first 2 characters always
  # indicate the state. See https://en.wikipedia.org/wiki/FIPS_county_code.
  # Breaking out the state allows for easier state selection later.
  # 2. This file has all counties, some of which do not have a ZCTA. Remove
  # those counties.
  zcta_crosswalk |>
    mutate(state_fips = str_sub(.data$county_fips, 1, 2)) |>
    filter(!is.na(.data$zcta))
}

The function reads and transforms the contents of a URL. At the time of this writing that is the URL for the Census Bureau’s “2010 ZCTA to County Relationship File”, a file which I mentioned earlier.

This means that if Census publishes an updated dataset in the same format tomorrow you could just change the URL, rerun the code and get the updated data in R. (Note that I do not know when Census plans to update this dataset or whether they plan to publish it in the same format.)

If you open the URL referenced in ?get_zcta_crosswalk in a browser you will see rows like this:

221704258470394|90210|ZCTA5 90210|27823432|153478|G6350|B5|S|275901063468976|06037|Los Angeles County|10513491099|1787501506|G4020|H1|A|27823432|153478

This tells us that ZCTA 90210 is in Los Angeles County. It also tells us that Los Angles County has FIPS Code 06037.

Adding in State Information

Unfortunately, the file does not directly contain any state information. And since I wanted to run queries like “Get all ZCTAs in a given state”, I needed to add that in.

I started by splitting out the first two characters of each County FIPS Code into a new column called state_fips. This allows a user to search for ZCTAs in a state if they know the state’s FIPS code.

However, this does not help us if users want to select a state by it’s name or Postal Code Abbreviation. To address this limitation I created a new dataframe called state_names, and used it to join against the results of ?get_zcta_crosswalk:

data(state_names)

print(state_names, n = 5)
#> # A tibble: 56 × 4
#>   full       usps  fips_numeric fips_character
#>   <chr>      <chr>        <int> <chr>         
#> 1 alaska     AK               2 02            
#> 2 alabama    AL               1 01            
#> 3 arkansas   AR               5 05            
#> 4 arizona    AZ               4 04            
#> 5 california CA               6 06            
#> # ℹ 51 more rows

One thing to keep in mind is that while there are technically only 50 states, “state” in this dataset really means “any top level administrative region”. This dataset contains 56 states (the extra ones are: the District of Columbia, Puerto Rico, US Virigin Islands, American Samoa, Guam and the Northern Mariana Islands).

I believe that it would be useful for R to have a standalone package that contains a data frame like this for all FIPS codes. I did not break state_names out into a separate package because even though it has 56 state-level entities, the full list is much larger.

The code I used to generate state_names is in inst/gen_state_states.R.

Note that while R has two built-in vectors that deal with state names (state.abb and state.name), they cannot help us here because: (1) they do not contain FIPS codes and (2) they only contain 50 states.