Zip Codes are Not Areas

ZIP codes are postal codes in the United States created by the US Postal Service. Perhaps the most common misconception in GIS is that Zip codes are polygonal regions or areas. People often think of mapping in the US as a hierarchy of ever-subdivided polygonal areas: states, counties, cities, zip codes. If they need higher resolution than a county, they next leap to zip codes because they think of zip codes as polygons. This is not true. 

 

Zip codes are linear features associated with specific roads or with specific addresses such as apartment buildings or military bases that are best regarded as a point. In some cases, Zip codes have no physical location because they are assigned to a mobile or abstract "location" such as a military ship.

 

Even in the most common case of Zip codes assigned to streets, Zip codes do not clump together in groups that may be covered by rational polygons. We can consider an example using a map of part of Reno, Nevada, shown below. This map is fairly typical of the situation in mid-sized urban areas. It is extracted from the US Census Bureau's TIGER/Line 1997 data set, which includes roads as segments of lines, with most line segments coded with Zip and Zip+4 codes for that particular segment. In this note, we will refer to both Zip codes and the Zip+4 extension together under the name "Zip code".

 

To create polygons from road lines where lines have a common zip code there are several approaches. One possible approach is to select all line segments with the same Zip code and to then draw an area (polygon) that encloses them. This can be done by creating a buffer zone about each street line having a particular Zip code and then doing a Union of the buffer zone areas thus created. The blue, purple and green areas were created in this way and each represent a a different Zip code value.

 

The road lines shown in red selection color all have yet another Zip code in common. Immediately there are three pathologies visible in this map.

 

images\sc_zip_pathology.gif

 

First, note that the blue area is not contiguous. Second, note that there are many regions of overlap between the blue and the purple areas and between the purple and green areas (we should have used varying layer opacity so that the regions of overlap were clearer). Third, note that at least one road segment highlighted in red (all having the same Zip code) occurs inside the purple zone where it is completely surrounded by all adjacent streets having a different Zip code.

 

The above situation is extremely common. In fact, we selected this particular map at random. Any urban map in the US will show similar, if not even more bizarre effects. Rural maps can have such a sparse network of roads with such strange zip code assignments that some rural areas cannot even be approximated with zip code regions.

 

For the above reasons, any map that purports to show "Zip Code Areas" or "Zip Code Polygons" should not be taken as a precise map showing Zip code locations. It is at best some sort of approximation and most likely is wildly inaccurate in certain regions. The approximations can be useful, but they should not be confused with the real thing.

 

The US Postal Service, of course, doesn't make it any easier to deal with such issues by making it easy to get Zip code information. Zip code information is not available for download via Internet from the US Postal Service. It is best obtained from (of all agencies!) the US Bureau of the Census.

 

ZIP Code Tabulation Areas (ZCTAs)

 

For statistical tabulation purposes the Census Bureau has long found it convenient to work with Zip code groupings of population. Zip codes have been so useful that the Bureau embarked on a project to create a standardized map of the US showing the approximate region of coverage of various Zip codes as areas. These areas are known as ZIP Code Tabulation Areas (ZCTAs). ZCTAs may be downloaded from the Census Bureau's www.census.gov site. Drill down to the Cartographic Boundaries pages to get ZCTAs. Download them using .e00 format so they will import into Manifold using the correct NAD83 datum.

 

Before ZCTAs were published, every vendor of maps used in GIS had to resolve the various ambiguities posed by Zip code pathologies like those shown above. With ZCTAs the GIS industry can now use a standard approximation that is the same used as the Census Bureau for publishing demographic information. It is not clear if the Bureau will continue to create ZCTAs after the year 2000; however, they are so useful we believe they will become the industry standard for maps representing Zip codes as areas.