ZIP Code Database Listings, Maps, and Boundary Data
Despite the fact that ZIP codes seem to be geographic in nature, that wasn't their intended purpose. The cost of mail was relatively much higher back then. As you can see from the map of the first 3 digits of zip codes , the digits after the first are also generally assigned from east to west. Remote and especially rural areas of the country do not have enough deliverable addresses to create a mail route. For cities, the assignment is somewhat more complicated. They weren't made to correspond to existing boundaries such as cities, counties, or even states.
ZIP Code Database
The cost of mail was relatively much higher back then. It cost twenty-five cents to send a letter miles — not much less than it costs today to send a letter anywhere in the country, and in considerably less time! Today, many of us in the United States expect to have our mail delivered to our homes and offices at no extra charge.
Before July 1, , city residents had to pay to have a postal worker bring their mail to them; rural customers had to travel to pick up their own mail for another 30 years. Zip codes wouldn't have helped much in those days. Many famous Americans have been postal workers. Truman was postmaster of Grandview, Missouri for a time. Perhaps the most romantic era in our United States postal history is that of the Pony Express. Before then, stagecoaches took more than 20 days to deliver mail from coast to coast.
In , William H. Russell bought strong horses and put a listing for good horseback riders in newspaper ads that read: Young, skinny, wiry fellows not over Must be expert riders willing to risk death daily. The service closed in when telegraph lines connecting the coasts were finished being laid, allowing people to send information much faster and cheaper than they could by Pony Express. The history of the United States Post Office is filled with fascinating data and amusing facts.
Did you know the first Post Office in the United States was actually a tavern? Or that the United States employed camels to deliver mail over deserts in the Southwest? In most cases, addresses in close proximity to each other are grouped in the same ZIP code which gives the appearance that ZIP codes are defined by a clear geographic boundary.
However, some ZIP codes have nothing to do with geogaphic areas. When ZIP codes appear to be geographically grouped, a clear shape cannot always be drawn around the ZIP code because ZIP codes are only assigned to a point of delivery and not the spaces between delivery points. In areas without a regular postal route or no mail delivery, ZIP codes may not be defined or have unclear boundaries. The main issue is discussed above: The Census Bureau and many other commercial services will try to interpolate the data to create polygons shapes using straight lines to represent the approximate area covered by a ZIP code, but none of these maps are official or entirely accurate.
They provide a very close approximation of the area covered by a ZIP code. You can easily notice some of the boundary issues when viewing our maps.
Very rural areas aren't labeled as belonging to a ZIP code such as much of Nevada and Utah where there are few, if any, addresses to deliver mail.
If the address is on the same street as a ZIP code boundary on the map, be sure to search for the full street address to determine the ZIP code instead of relying on the map. Their purpose is to convey statistical data about regions that are familiar to most citizens. As discussed above, it is difficult to precisely define a geographic area covered by a ZIP code. ZCTAs were developed to account for some of the difficulties in assigning an area to a ZIP code and to precisely define a geographic area.
In general, they are updated once every 10 years for the Census. The Census assigns an area to a ZCTA according to census blocks the smallest geographic unit used by the census.
Imagine a city block that makes up a typical census block as pictured to the right. It is bounded on all 4 sides by portions of city streets that each have their own name and addresses.
The issue is that census blocks almost always split down the middle of the street. ZIP codes rarely do because that would require two postal workers delivering mail to that street - one for each side of the street. In the example, one mail carrier may deliver to 3 sides of the block via one ZIP code while another mail carrier delivers mail on the other street in a different ZIP code.
When this happens, the Census Bureau will assign the entire block to a single ZCTA in this case, because the census block is the area that is precisely measured. If you are getting very precise usually a matter of meters, not miles , census block boundaries near the edge of a ZIP code almost always split ZIP codes. Remember that ZIP codes were made to make mail delivery easier. They weren't made to correspond to existing boundaries such as cities, counties, or even states.