Robb Shecter of the Weblaws.org site wanted to map out restaurant health inspections in Multnomah County, Oregon as part of his efforts to create apps to open up government information by increasing access. The data from the county on each restaurant doesn’t always contain place information such as the city name. What Shecter found however, is that the ZIP code is always listed on the health inspection records for each restaurants.
In order to map out the restaurant locations, Shecter needed a list of ZIP codes associated with each city name. In order to obtain the list of ZIP codes, he sent a Freedom of Information Act (FOIA) request to the United States Postal Service (USPS) for “an electronic copy of all records of current ZIP Codes and each Code’s corresponding preferred city and state.”
The USPS’s response was to deny the request, citing subsection (c)(2) which exempts it from the disclosure of “information of a commercial nature, including trade secrets, whether or not obtained from a person outside the Postal Service, which under good practice would not be publicly disclosed.” James Wilson, Manager of Address Management, who wrote the response letter from the USPS, the further states, “I have determined that the information you have requested is commercially sensitive, proprietary business information that is protected by FOIA Exemption 3 and 39 U.S.C. §§ 410(c)(2).”
Robb Shecter has posted PDFs of both his initial request letter and the denial response from the USPS on his Weblaws site: USPS: ZIP Codes are “Commercially Sensitive” Trade Secrets
An interesting side note mentioned in the comments is that, ZIP codes don’t represent geographic areas but more closely represent mail carrier routes. Most people think of ZIP codes as a polygon file, with the ZIP code representing a distinct area. There are some helpful explanations for what ZIP codes are on this Stack Overflow discussion about how to get the bounding box of a ZIP code. In the article on GIS Lounge, “Understanding and Mapping ZIP Codes” the US Census definition states that ZIP codes may refer to a “street section, a collection of streets, an establishment, a structure, or a group of post office boxes.”