No doubt you're familiar with one or more popular online mapping services. How well do they do at geocoding the location of a postal address? You can try it out for yourself at several web-based mapping services, including MapQuest.com, Microsoft's Bing Maps, and Tele Atlas/TomTom's Geocode.com (no longer a live site). Tele Atlas, for example, has been a leading manufacturer of digital street data for vehicle navigation systems. To accommodate the routing tasks that navigation systems are called upon to serve, the streets are encoded as vector features whose attributes include address ranges. (In order to submit an address for geocoding at Geocode.com, you have to set up a trial account through their EZ-Locate Interactive web tool or download the EZ-Locate software).
Shown above is the form by which you can geocode an address to a location in a Tele Atlas street database. The result is shown below in Figure 4.9.2.
Let's compare the geocoding capabilities of MapQuest.com to locate the address on an actual map.
The MapQuest.com map from 2013 estimates the address is close to its actual location. Below is a similar MapQuest product created back in 1998. On the older map, the same address is plotted on the opposite side of the street. What do you suppose is wrong with the address range attribute in that case?
On the map from 1998, also note the shapes of the streets. The street shapes in the 2011 map have been improved. The 1998 product seems to have been generated from the 1990 version of the TIGER/Line files, which may have been all that was available for this relatively remote part of the country. Now MapQuest licenses street data from a business partner called NAVTEQ.
The point of this section is to show that geocoding with address ranges involves a process of estimation. The Census Bureau's TIGER/Line Shapefiles, like the commercial street databases produced by Tele Atlas, Navigation Technologies, and other private firms, represent streets as vector line segments. The vector segments are associated with address range attributes, one for the left side of the street, one for the right side. The geocoding process takes a street address as input, finds the line segment that represents the specified street, checks the address ranges to determine the correct side of the street, then estimates a location at the appropriate point between the minimum and maximum address for that segment and assigns an estimated latitude/longitude coordinate to that location. For example, if the minimum address is 401, and the maximum is 421, a geocoding algorithm would locate address 411 at the midpoint of the street segment.
Try one of these geocoding services for your address. Then compare the experience, and the result, with Google Maps, launched in 2005. Apply what we've discussed in this chapter to try to explain inaccuracies in your results, if any.