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What is Geocoding?
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
2. Intersection Matching
3. ZIP Code Centroids
Geocoding uses address interpolation and intersection matching for geocoding addresses in the United States, Great Britain, Belgium, Netherlands, Luxembourg and parts of Canada, Denmark, France, Germany, Italy, Spain, Sweden and Switzerland. ZIP Code centroids are available only within the United States.
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. 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 249 Jackson Street would plot in the middle of a Jackson Drive segment with an address range of 1-99 on one side and 2-100 on the other.
Address Interpolation Failures
There are several cases where address interpolation will fail, including:
1. Ambiguous addresses -- 50 Jackson Drive and 50 W Jackson Drive both exist.
2. New addresses -- where the address ranges haven't yet been added to the data provider's database.
3. Vanity addresses -- where the address cannot be located on a street segment, such as #1 Town Center Mall.
4. 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.
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.
Intersection Matching. One of the easiest and perhaps most accurate methods of geocoding a location is using an intersection. Using the same U.S. postal data files described above, intersection matching locates a node that is common to two street segments. The latitude and longitude coordinate of the common node is returned.
Geocoding procedure finds the best available coordinate for an address by attempting to geocode an address in the in the following sequence:
1. Address Interpolation
2. ZIP+4 Centroid
3. ZIP+2 Centroid
4. 5-Digit ZIP Centroid
The most accurate geocode will be returned. If the location that is displayed on a map appears to be quite different than the expected location, it was probably based upon one of the ZIP centroids. Using as complete an address as possible (such as proper street name, city, state, and ZIP Code) will increase the chances of a more accurate geocode.
The alternative is providing the closest cross streets in the address field for an intersection match. Geocoding Geocoding Geocoding