Google spends a lot of time correcting data it receives, through a project called Ground Truth. Here, the left side shows the original data from the U.S. census program. At right is Google's corrected version.
The essence of the Ground Truth project is marrying Google's real-world information with data sets from sources such as cities, census operations, and postal address databases. "You can conflate those two together and correct one with the other," says Michael Weiss-Malik, engineering director for Google's Ground Truth and Map Maker.
Newer Ground Truth work involves mapping interiors such as the San Francisco Airport -- including routes through them. Buildings have triple the information density as roads, posing more processing challenges for Google.
Part of the Ground Truth effort is correcting bad geographic data from sources such as governments. In cases where the data set is correct but distorted or shifted, Google can mathematically translate geographic items to new locations.