Crime Mapping Snafu Caused by Erroneous Geocoding

LAPD crime logoThat aggravated assault isn't where you think it is. Due to an error with its geocoder, the Los Angeles Police Department mapped as many as 4 percent of its crimes at a single point, even if the real location was miles away.

The fault actually lies with the contractor who built the site, or a sub-contractor who provided the geocoder. Whenever an address or intersection couldn't be converted into coordinates, the center of Los Angeles was used as a default location. Likely this was caused by the geocoder passing the most precise data possible--in this case, city-level. Of course, a crime map isn't much use without street-level precision (and crime is a popular mashup theme, with over 40 crime-related mashups listed in our directory).

LAPD Crime Map

The error made it outside the LAPD site, as well. Local news aggregator EveryBlock lists nearly 2,000 crimes at that one location. For example, this aggrevated assault took place at "Ocean Front Wk and Washington Bl," near Venice Beach, 17 miles away from its geo coordinates.

This error is a good reminder that mashups can only be as good as the data that feeds them. The LA Times discovered the problems while working on its own crime site, only after viewing six months of aggregated data. It's not always possible to get beyond a sanity check for errors. In the case of geocoding, both Yahoo and Google's geocoders provide a precision level. It's a good idea to check that it matches what you expect.

[Hat tip: CNET]

Be sure to read the next Mapping article: Microsoft Virtual Earth API Gets Improved 3D, Geocoding, and Performance