<P> ZIP Codes also change when postal boundaries are realigned . For example, at the same time at which the above - noted change in Montgomery County, Maryland, took place, and under pressure from then - mayor of Washington, D.C., Marion Barry, the USPS realigned the postal boundaries between Washington, D.C. and Maryland to match the boundary . Previously, many inner suburbs, such as Bethesda and Takoma Park, Oxon Hill, Temple Hills, Suitland, and Capitol Heights had been in the Washington, D.C., postal area . As a result of the change, ZIP Codes in Maryland beginning with 200 were changed to new ZIP Codes beginning with 207, 208 or 209, depending on their location, and ZIP Codes straddling the D.C. - Maryland line were split . For example, 20016 (Bethesda) became 20816, while the Maryland portion of 20012 (Takoma Park) became 20912 . </P> <P> Delivery services other than the USPS, such as FedEx, United Parcel Service, and DHL, require a ZIP Code for optimal internal routing of a package . </P> <P> There are over 42,000 ZIP Codes in the United States . ZIP Codes are used not only for tracking of mail but also in gathering geographical statistics in the United States . The U.S. Census Bureau calculates approximate boundaries of ZIP Codes areas, which it calls ZIP Code Tabulation Areas (ZCTAs). Statistical census data is then provided for these approximate areas . The geographic data provided for these areas includes the latitude and longitude of the center - point of the ZCTAs . There are approximately 32,000 ZCTAs . The reason that there is not one ZCTA for every ZIP Code is that PO Boxes are excluded, since only populated areas are included in the Census data . The Census Bureau provides many statistical data sets for ZIP Codes, but does not keep up - to - date datasets of all ZCTAs . Complete datasets providing a similar approximate geographic extent are commercially available . </P> <P> ZIP Codes are inherently discrete data, or point - based data, as they are assigned only at the point of delivery, not the spaces in between the delivery points . The U.S. Census Bureau then interpolates this discrete data set to create polygons, or areal features representing the approximate extent of the ZIP Code to use for mapping and data presentation . ZCTAs are not to be confused with ZIP Codes, and they are not updated as frequently as ZIP Codes . However, for many research and planning purposes, they are very useful and can be used with ZIP Code data . </P>

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