Geocoding assigns a latitude/longitude pair to a street address. The result is not exact -- geocoding creates the best possible estimate based on available information. MapQuest uses one of several methods to arrive at a geocode.

Address Interpolation

This method provides the most accurate results. In address interpolation, streets are broken up into segments, and each segment has a range of addresses associated with it. MapQuest's software makes an educated guess at the location of a given address based on where it should be located within that segment. For example, let's say ask MapQuest to show you the location of "25 Elm Street, Hollywood, CA." A geocoding server will ask a mapping server for the best available data for Elm Street in Hollywood. It will then locate the segment with addresses ranging from, say, 1 to 49 on one side and 2 through 50 on the other. It will assume that 25 is at the center of the segment on the odd-numbered side of the street, and it will locate the address at the latitude/longitude coordinates for that position.

If it can't geocode an address using interpolation -- typically because the street isn't in its database yet or there are a number of streets with the same name in a given area -- the server will attempt to use either intersection matching (if the address is outside of the United States) or zip code centroids (if the address is in the U.S.).

Intersection Matching

To geocode an address using intersection matching, MapQuest finds two street segments: one segment near where MapQuest thinks the address should be located, and one that touches that segment. It then geocodes that intersection, which it assumes to be the closest intersection to the given address. In this case, MapQuest delivers a map pinpointing the intersection or directions to the intersection.

Zip Code Centroids

In the United States, every address has a "zip code" that corresponds to a particular region within a city. A zip code centroid is an area that corresponds to a five-digit, seven-digit or nine-digit zip code. Most of us are familiar with the five-digit zip codes, but those zip codes are actually broken up into smaller, zip+2 areas and even smaller zip+4 areas. The zip+4 centroid is the nine-digit zip code you sometimes see on official documents, and it typically includes only about 10 homes (you can look up the zip+4 code for your address using the USPS ZIP Code Lookup). If MapQuest can figure out which +4 centroid the address should be in, it will calculate a latitude/longitude pair that is at the center of that centroid. The next step is to try to place the address in a +2 centroid, and finally in the basic five-digit zip code area, which is quite large. These latter options don't return very accurate results.

Now that we know how MapQuest generates maps and directions, let's find out how we can use to get the information we want.