OpenStreetMap uses ground truth
Surveys and personal knowledge
Ground surveys are performed by a mapper, on foot, bicycle, or in a car, motorcycle, or boat. Map data was typically recorded on a GPS unit. In late 2006  Yahoo! made their aerial imagery available for tracing to OpenStreetMap contributors, which simplified mapping of readily visible and identifiable features. The project still makes use of GPS traces from volunteers which are used to delineate the more difficult to identify and classify features such as footpath, as well as providing ground-truth for aerial imagery alignment.Once the data has been collected, it is entered into the database by uploading it onto the project's website together with appropriate attribute data. As collecting and uploading data may be separated from editing objects, contribution to the project is possible without using a GPS unit, e.g. by using Paper mapping.Some committed contributors adopt the task of mapping whole towns and cities, or organising mapping parties to gather the support of others to complete a map area. A large number of less active users contribute corrections and small additions to the map.