County Map Boundary Data Frequently Asked Questions
County Boundary FAQs
What SRID/Datum is Used?
All of our boundary data uses SRID 4326 or WGS84.
How is County GIS data created?
Using sophisticated algorithms that strictly interpret monthly data feeds from the US Postal Service, our ZIP Code GIS data is built from the address up to represent on-the-ground reality. Each boundary is closely tied to the street network as defined in the TIGER/Line® street data network.
Why do ZIP Code boundaries have to be updated?
There are about 45,000 5-digit ZIP Codes in the US. The exact number changes frequently. Most of those changes are quite small. Others are significant.
ZIP Codes change for two reasons: population growth requires adding ZIP Codes or ZIP Codes are reorganized to facilitate mail delivery.
How much of the country does your ZIP Code data cover?
Our ZIP Code GIS data covers the entire United States.
Doesn't the USPS make ZIP Code maps?
No. ZIP Code maps are not made with polygons but with collections of deliverable addresses. Boundaries have to be created using those address clusters and given latitude and longitude coordinates. This is an extensive process that falls outside the mission of the USPS. In fact, USPS.com refers all visitors looking for ZIP Code maps to uur data.
In what ways are your ZIP Codes more accurate than other ZIP Code datasets that are available?
The enormous effort and amount of resources required to create accurate ZIP Codes often entices companies to take short-cuts. The fact is that some ZIP Codes are easy to map and others are difficult in that they require thought and research. Many data providers over-generalize boundaries by simply dropping ZIP Codes that are hard to map.
We never takes those short-cuts. Strict data methodologies and quality assurance checks ensure that ZIP Code Boundaries accurately capture all ZIP Codes, not just those that are easy to map.
What is a ZIP Code?
ZIP is an acronym for Zone Improvement Plan, and refers to the USPS’ initiative in the 1960’s to improve the sorting and delivery of mail. Units with roughly the same number of deliverable addresses were created and termed ZIP Code.
ZIP Codes are comprised of 5 numbers. An additional four, referred to as ZIP +4, are appended by the USPS to allow mail to not only reach the postal town associated with a ZIP Code but also be sorted to the level of city block, office building, or individual high-volume receiver of mail.
Why are there filler ZIP Codes? How can they be used?
Filler ZIP Codes are utilized by us to help create continuous polygonal ZIP Code coverage across the United States in places where there isn't sufficient information to define a ZIP Code point or polygon. Filler ZIP Codes can be used for display purposes so that visually there are no holes. Filler ZIP Code structure is as follows: a 3-digit ZIP Code followed by either an "MH" or "MX" (ex. 901MH) to differentiate areas encompassed by water (MH) or land (MX).
Can ZIP Codes cross state boundaries? If so, how are the polygon represented in this product? Is the same true for county boundaries?
ZIP Codes can cross state boundaries. In our ZIP Code Boundaries product these ZIP Codes are represented as one polygon and are assigned to the state where the centroid of the ZIP falls. The same is true for ZIP Codes that cross county boundaries.
Why do some ZIP Codes have multiple polygons?
ZIP Code data is not really polygonal. ZIP Code data can be non-contiguous pockets of addresses served by a specific zip code. Additionally, topological features such as water (a bay or river - especially with islands) or mountains can create address pockets. ZIP Codes are created solely for the purpose of efficient mail delivery which results in these kinds of quirks and oddities.
