I recently downloaded ZCTA boundaries to replicate work I previously did at the tract level, but I am finding that the ZCTA boundaries do not include any information about the state or county for each ZCTA. This is different from the tract files, which include state and county information, making it very easy to filter to the study area I need. With the ZCTAs, I have no convenient way of filtering the data to my area of interest.
Is there a reason for this discrepancy? And is there a recommended way to attach the state/county information to the ZCTA boundaries?
Thanks.
Census Bureau TIGER/Line files are the primary source for NHGIS boundary files, and they include higher-level codes only for geographic entities that officially and uniformly nest within another level’s entities. ZCTAs do not nest within any other level of census geography. It is possible for ZCTAs to cross county lines and even occasionally state lines. Census tracts, on the other hand, nest within both states and counties. The Census Bureau provides a diagram of the nesting relationships among standard census geographic entities through this page.
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To get associations between ZCTAs and other census geographic levels–or between any pair of non-nesting geographic levels–I recommend using the Missouri Census Data Center’s Geocorr site.
In addition, many of the applications that allow you to read and work with shapefiles also have support for “spatial joins”… linking one geographic layer to another by spatial relationships. If you obtain state or county boundary data along with ZCTA boundaries, you could use a spatial join or other spatial selection operations to select the ZCTAs that intersect a selected set of states or counties.
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Thank you, this was very informative! I had no idea that ZCTAs were not just a normal in-between of counties and tracts until I started running into this problem. I actually was trying to filter the ZCTAs by state and county boundaries, but I kept getting ZCTAs outside state borders… I suspect this is explained by your comment; there must be some ZCTAs that cross my boundaries. I was planning to transition to ZCTAs from tracts based on comments from a committee member, but after looking into this a little further, it seems that tracts are more standardized geographical unit that better correspond to real neighborhoods.
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