how the world's biggest open map stays correct
Anyone can create an account on OpenStreetMap and start editing immediately.
This means that some people with bad intentions make malicious edits, creating places that don't exist, deleting places that do, or adding incorrect information.
To combat vandalism, OpenStreetMap superusers track recent changes constantly and flag changes that look wrong.
They can then revert - or undo - incorrect changes and notify the user. Users that repeatedly vandalize the map can have their editing rights suspended or removed entirely.
Since there's no perfect map of the world to compare ours to, finding areas where data is missing is difficult.
Data is often partial - a road is present, but unnamed, or a building lacks basic details that let it be categorized.
Problems with this routing network are invisible to other users of the map, and can include disconnected roads, improper turn restrictions, and unmarked speed limits.
This leaves open the question: how good is OpenStreetMap data? There are a few solid investigations of this question to point to.
A Quality Analysis Of OpenStreetMap by Aamer Ather at University College London concluded
The results of this analysis found the positional accuracy of OpenStreetMap data to be very good in comparison to OS MasterMap, with over 80% overlap between most the road objects tested between the two datasets. The results also found there to be a positive correlation between road name attribute completeness and number of users per area.