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Every decade the US Census Bureau releases a dataset called “TIGER” that includes what it calls “ZIP Code Tabulation Areas” or ZCTAs. As the Census Bureau itself admits, ZCTAs are not ZIP Codes. They are simply areas to help the Census staff go out and collect/analyze census information.

“There is no correlation between U.S. Postal Service ZIP Codes and U.S. Census Bureau geography.... The geographic entities the Census Bureau uses to tabulate data by are relatively stable over time. For instance, census tracts are only defined every ten years. In contrast, U.S. Postal Service ZIP Codes are designed to meet the day-to-day operational needs of the U.S. Postal Service and tend to change more frequently than every ten years.”

From www.census.gov on October 2, 2007

Many companies have tried to use ZCTAs instead of ZIP Code polygons and found that since the data is so old and inaccurate, it causes more harm than good. Remember, the ZCTA polygons you can get from the Census Bureau are 9 yrs old. Of course, the price can’t be beat – the data itself is free. But free data that is wrong ends up costing plenty.

The images at http://www.maponics.com/GIS_Map_Data/ZIP_Code_and_Carrier_Route_GIS/ZCTAs_vs_ZIP_Code_Data/ZCTA_vs_ZIP_Code.png illustrate concretely some of the differences between ZCTA data and current ZIP Code data. All of the ZIP Codes highlighted yellow in the ZIP Code data above do not appear at all in the ZCTA data below. These are ZIP Codes that have been added since the last ZCTA update. Some ZIP Codes that are included in both data (for example 85242 in the bottom right corner), have changed shape over time, as new ZIP Codes have been

About the Author:

The author, Darrin Clement, works with Maponics, which focuses on map data licenses for neighborhood boundaries and ZIP Code maps. You can find out more at www.maponics.com.