The Birth of geocoder.us

Strangely enough, the removal of useful features from online map services seemed to occur right before a surge of interest in free sources of geodata among the free and open source software community.

Collecting this data and keeping it up to date with "ground truth squads" who go around and verify that streets are where they are supposed to be and that houses haven't up and run off, is quite expensive.

An alternative to the full expense of this data lies in the U.S. Census Bureau. They have compiled TIGER (Topologically Integrated Geographic Encoding and Referencing system) data. TIGER data is used as part of the normal fulfillment of their duties to do an actual enumeration of the people every 10 years. This data is imperfect, but the regular tasks of census workers are similar to our own needs. They wish to identify the location of a residence based on a street address, just as we do when we geocode.

Again, it is important to stress that TIGER data is imperfect, however "imperfect but free" has its own charm. TIGER data is also used as the basis for the free TIGER Map Server offered by the Census Bureau at http://tiger.census.gov/cgi-bin/mapsurfer.

There is a lot of interesting information about geography and the challenges of capturing complex and inconsistent information to be found in the TIGER documentation. But for simple geocoding, all you really need to know is that the TIGER data endeavors to include information on every street segment in the U.S. For each block, the TIGER data includes the street name, the latitude and longitude at each end of the block, and the range of address numbers for the left and the right side of the street.