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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One major difference between ZCTAs and ZIP codes is that houses on either side of a street will usually be in the same ZIP code, but they could be in different ZCTAs, because block boundaries run down…
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…
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
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 ?
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.
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”