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.

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.

In order to map address location you need to have two files. You need a table that contains the addresses you are interested in mapping. It is also essential that you have a reference file that contains location information about street names, address number ranges, and zip codes.

When looking for address locations, reference maps are usually streets maps where each segment contains attribute information about the street name, the address ranges on the right and left side of the street, and the zip code.

When geocoding addresses, your GIS systems will find the street and address location of the line segment in your reference map that matches the street address in your table.

When your GIS system finds the line segment with the right name and address ranges it will place a point. Point placement is based on where the address number falls in the address range.

1. Encountered When Geocoding

When you start geocoding you might not find all the address locations right away. Unmatched address locations are due to errors in the reference map or the address database itself. Below is a listing of common problems associated with either the reference map or the base address map.

2. Associated with the Reference Map

Missing Address Ranges (Tiger Files often have Missing Address Ranges)
Gaps in Address Ranges
Missing Zip Codes
Incorrect Zip Codes
Use of highway numbers instead of name. (e.g. Highway 2A instead of Massachusetts Ave.)

3. Associated with Address Tables

Misspelled street and town names
Missing or inaccurate address numbers s
Using a spelled-out address number: "One Main St." instead of "1 Main St."
Addresses that are not traditional street addresses:
1 Cambridge Center
1 Kendall Square
10 Post Office Square
More than one name for a street:
"Mass. Ave." or "Massachusetts Ave." or "State Hwy. 2A"
"I - 90" or "Massachusetts Turnpike" or "Mass. Pike"
"O'Brien Hwy." or "Msgr. O'Brien Hwy" or "Monsignor O'Brien Hwy."
"Glynn Way" or "Theo A. Glynn Way"
"3 St." or "3rd St." or "Third St."