ACS Data Users Group

Hi all,

Does anyone know if there is a zip code/zip code tabulation area crosswalk (i.e. which zip codes are consolidated into a ZCTA). I know that certain zip codes are omitted from ZCTAs for a number of reasons (e.g. large unpopulated areas of land https://www.census.gov/geo/reference/zctas.html).

Specifically, I am looking at the ZCTAs for Los Angeles County, California. I have a list of the zip codes for LA county and pulled up the ZCTAs for California. When I compared the zip codes and ZCTAs, I saw that 68.7% of the zip codes did not have a matching ZCTA. Is there any way to find out if some of the zip codes were combined with a particular ZCTA?

Thanks, in advance.

Cole

Speaking for data in Michigan, I've never seen a ZCTA which combined zip codes. When a zip is missing, it's usually because either it's a PO box or it's totally non-residential. Many large companies have their own zip codes.

That said, 69% seems awfully high.

Patty Becker

Thanks for chiming in, Patty. Good to know that for certain locations the ZCTA matches well with residential zip codes. I agree with you that 69% seems high.

I can tell you a few things. 1. Well, you know, there are NO official borders to ZIP codes. I tried to find some. Not even the post office could really tell me. 2. There are no standard generally agreed on borders. I looked at different websites and got different borders. Sometimes one website had the ZIP code in a completely different area than did another website. But folks already know that. So 3. if you go to the US Census site, you can get borders for ZCTAs. Then you can use some software like ESRI to pull in those US Census ZCTA borders, and a lot of those GIS programs will have their own set of borders for ZIP codes. Pull those up too, and use the software to compare borders and identify where any ZIP codes might be combined into any particular ZCTA. Hope that helps.

Thanks Gene! I appreciate you confirming the fluidity of zip codes and for pointing out a method to compare the boundaries between a ZCTA and a zip codes, very helpful!

Try this. Go to the link below and weed out the ones on your list that are POBoxes and "Unique" (the "unique" are pretty much the businesses/universities that have their own zip code).
www.zip-codes.com/.../ca-los-angeles.asp

I dont think you'll find a crosswalk because of the changing nature of zip code boundaries. They are somewhat "fluid" because zip codes are created by the USPS for the purpose of delivering mail, so their boundaries change as carrier routes get adjusted.

If you haven't already, it may also be helpful to read these items about ZCTAs and how they are created and how they compare to zip codes.
How ZCTAs compare to zip codes:
www.census.gov/.../zcta.html

ZCTA documentation:
www.census.gov/.../zctas.html

We are fortunate in SE Michigan because about 20 years ago the ZIP codes were redone in some counties and aligned with our city/township (MCDs here) geography. That makes the boundaries consistent. However, it's certainly true that often it's unclear where a zip code boundary actually is. But...Census can define them through the reported zip codes of addresses on the MAF, so that should generally lead to consistency even when they're hard to map.

In general, addressing throughout the nation has improved greatly in the past 25-30 years because of the 911 systems' requirements. That's why rural areas got addresses instead of rural route/box numbers. The ambulance has to be able to find the house!

Patty, you wrote "But...Census can define them through the reported zip codes of addresses on the MAF"
Are you saying the US Census Bureau can define boundaries of ZIP codes? I know they have boundaries for ZCTAs, but having boundaries for ZIP codes, I didn't know that.

Gene - well, yeah, they can outline them to a close approximation. For ZCTAs, they round them to block boundaries because the data are tabulated from blocks, not from electronic maps. I don't think they've ever published those lines, but I'm sure they know where they are and where they overlap. But that's why I don't worry about the correspondence between Zips and ZCTAs.

Thank you for sharing these resources. Going through the list of zip codes and omitting those which are PO Boxes or Unique will help me a lot, thank you! I also appreciate you pointing me to those ZCTAs reports. I had read the second link prior, and found the youtube video a helpful refresher.

JamiRae thanks again for your suggestion. I went through that website and compared the ZCTA list with those zip codes and you were right. Most of the missing ZCTAs were for PO Boxes and Unique zip codes. Thanks again!!