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.