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ZCTA vs. zip codes?

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12-17-2015 06:50 AM
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SallieVaughn
Occasional Contributor

​I understand that ZCTA's and zip codes can be different in places. The ZCTAs represent zip code aggregations at the block level. Does anyone know if there is any way to determine which blocks had data points with more than 1 zip code?

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ChrisSmith7
Frequent Contributor

Just in case anyone is interested, here's an animation of the process:

ZIP Code Tabulation Areas (ZCTAs) Delineation Animation - Geography - U.S. Census Bureau

I couldn't find a relationship file to give you this information, but, you could acquire a block group layer and a regular zip layer. When you intersect the two, you can find this information.

ArcGIS Help 10.1

ChrisSmith7
Frequent Contributor

I was thinking, though, this wont be 100% - the animation shows address points (with differing zips)... it's possible there are zip codes that wont be represented in the zip layer, which can and does happen.

In other words, say a block group shows an association with two zips. Your process would identify this, so you could say block group with GEOID xxxxxxx is associated with multiple zips. However, it's possible you could have other block groups that are only associated with one zip code, but, the post office maintains another zip code, maybe for an office building, or something like that. If that zip isn't represented in the zip layer, your process wouldn't identify this...

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JoeBorgione
MVP Emeritus

Zip codes are a different beast;  the USPS treats them as route features and we in GIS community view them as polygon features.  Personally, I can live with the amount of error the difference in perspectives gives.

That should just about do it....
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SallieVaughn
Occasional Contributor

I appreciate your take on accepting the error, and would be fine with that myself. However, a local home builder pointed out this discrepancy to us and come to find out, an entire subdivision is affected. According to him, there is a perception of lifestyle and safety associated with one zip code vs. the other. I hate to tell him, but the house is still in the same place

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JoeBorgione
MVP Emeritus

...hate to tell him, but the house is still in the same place...

Exactly!

That should just about do it....
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SallieVaughn
Occasional Contributor

I did find two resources which provide the correct zip code. Unfortunately, neither are GIS data.

https://tools.usps.com/go/ZipLookupAction!input.action?mode=1&refresh=true is a tool via USPS which allows you to check if a zip code is correct to a specific existing street address. Note that the USPS tool Every Door Direct Mail shows ZCTAs.

Free ZIP Code Map - US DataMart shows the most accurate zip code boundaries (verified by a single local post-master).

With these resources, we'll just modify our process flow for addressing to include a check of these two resources. We may also look into purchasing GIS data for zip codes if we can do so with a regional government collective and share the costs. However, it would be nice to visually display the census blocks which fall under this category so we can manually check fewer addresses with the resources listed above.

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