Zip Code Population by County

HI,  My county shares several zip codes with other counties.  How can I best determine the population for the portion of the zip code that is only in my county?  Thanks!

 

    

Marti Morris

Data Analyst

Public Health

 

Phone: 704.853-5097 

Mobile: 704-996-0422

 

991 W. Hudson Blvd.

Gastonia, NC 28052

 

www.gastongov.com

 

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Parents
  • Using Geocorr at the University of Missouri I got what I think you're looking for by using ZCTA as the "source" and county as the "target" example below, link is

    mcdc.missouri.edu/.../geocorr2022.html

    ZIP census
    tabulation
    area
    ZIP Code name County code County name Total population
    (2020 Census)
    zcta-to-county
    allocation
    factor
    21771 Mount Airy, MD 24013 Carroll MD 13,276 0.4325
    24021 Frederick MD 13,868 0.4518
    24027 Howard MD 3,172 0.1033
    24031 Montgomery MD 381 0.0124
Reply
  • Using Geocorr at the University of Missouri I got what I think you're looking for by using ZCTA as the "source" and county as the "target" example below, link is

    mcdc.missouri.edu/.../geocorr2022.html

    ZIP census
    tabulation
    area
    ZIP Code name County code County name Total population
    (2020 Census)
    zcta-to-county
    allocation
    factor
    21771 Mount Airy, MD 24013 Carroll MD 13,276 0.4325
    24021 Frederick MD 13,868 0.4518
    24027 Howard MD 3,172 0.1033
    24031 Montgomery MD 381 0.0124
Children
  • Dear Marti,

    I'm not sure how Geocorr works.  Since I use R I download the mapping data and check for the tracts (or block groups in this case) that have an intersection with the chosen ZCTA geography.  This method uses only the "raw" mapping data so it is pretty complicated. Once I have the tracts (or block groups) that intersect the larger geography I add up the populations for the individual tracts (or blockgroups). Likely not too helpful  in this case.

    Dave

  • Geocorr works by reducing both source and target geographies into collections of blocks -- the "atoms", if you will. We know the population of every block. So we can take all the blocks that are in ZCTA 21771 (say) and add up the populations of all those blocks that are also in county #24013, divide by the total ZCTA population to get the allocation factor, then repeat for any other counties that are partially covered by that ZCTA. That's why the allocation factors should always add up to 1, although there are sometimes rounding errors.

  • Yes entirely possible to do it yourself with block definition files and block populations. Did it in Florida after 2000 census to find the 1990 footprint of some  areas, for comparison, that had changed radically in 10 years. 
    it makes more sense now than it will in 5 years as change continues

  • Dear Glenn,

    I'm curious does Geocorr give you the fips codes for the "common" block groups ? What about statistics other than the total population ?

    Dave

  • Just as a comment, if you use my "brute force" method you can't just check for a "non-null" intersection between polygons. Some "adjacent" polygons may have a non-zero intersecting area because of an intersection along the border of two adjacent polygons.  I think that i allow 400 sq meters or so of overlap without including the polygon within the the larger polygon.

  • Hi Tim,  Thank you so much for your reply - tremendously helpful!  This is a great resource, and I sincerely appreciate the opportunity to expand my knowledge of ACS data.  Can't thank you enough!

  • Thank you for taking the time to reply!  Since I'm not familiar with R, this is a little beyond my skillset, but I appreciate the concept as you've described it.

  • Geocorr doesn't provide a list of the blocks in common between the source and target geographies, if that's what you mean, although it could be tweaked to do so.

    As for other statistics -- Geocorr wasn't designed to be a data tool. You can get total population, land area, and centroids, but that's all.

  • Since this is based on blocks you could get all the things counted by block in decennial census (subject to differential privacy) — detailed race/Hispanic, two age groups. Not from GeoCORR necessarily but the PL94 block data once you get the block definitions for your county/ZCTA

    as I say the passage of time since 2020 can distort things — in Florida there were giant blocks extending across the unpopulated Everglades to pick up a few scattered people, development occurred at the edges and created many populous tracts from one piece of that “block”