Geocoding is the process that assigns a latitude-longitude coordinate to an address. Once a latitude-longitude coordinate is assigned, the address can be displayed on a map or used in a spatial search.
There are three basic methods of calculating a geocode:
1. Address Interpolation-- Addresses within the United States are geocoded using software that accesses U.S. postal data files. These files contain street segments with address ranges attached to each side of the segment. Our geocoding program estimates the location of an address based on the length of the segment and the address range assigned to the segment. For example, an address of 49 Main Street would plot in the middle of a Main Street segment with an address range of 1-99 on one side and 2-100 on the other.
2. Intersection Matching-- There are several cases where address interpolation will fail, including:
Ambiguous addresses -- 50 Main Street and 50 W Main Street both exist.
New addresses -- where the address ranges haven't yet been added to the data provider's database.
Vanity addresses -- where the address cannot be located on a street segment, such as #1 Town Center Mall.
FPO, APO, or PO Boxes -- again, the address cannot be located on a street segment.
If an address interpolation cannot be made and the address is within the United States, the address will be geocoded using ZIP Code centroids.
3. ZIP Code Centroids--
If address interpolation fails, the geocoding program will attempt to assign coordinates to an address based on the ZIP code for the address. When possible, a nine digit ZIP Code centroid (also known as the ZIP+4) is used. This centroid is usually compact and averages 9 households.
The next two layers of precision are the ZIP+2 centroids and the 5-digit ZIP Code centroids. The ZIP+2 and the 5-Digit ZIP Codes vary widely in geographic size based upon natural features (such as rivers and mountains) and population densities. So the accuracy of a ZIP centroid geocode will vary based upon the quality and currency of the available data.