From OpenStreetMap Wiki
Ground truth has several possible meanings:
- "Ground truth" is a phrase used in cartography and sometimes within the OpenStreetMap community, to mean collecting data on the ground, on location. More specifically it can refer to doing this in order to get reference points for positioning aerial imagery data (or data traced from it). You might describe this as "ground truthing" the data. See also Good_practice#Map_what.27s_on_the_ground, quality assurance and accuracy.
- GroundTruth initiative (groundtruth.in) is a small technology consulting company directed by Mikel Maron and Erica Hagen, working on OpenStreetMap mapping and citizen journalism projects in poor and marginalized regions throughout the world, such as http://mapkibera.org
- "Ground truth" is also some kind of internal codename for mapping party ideas by the Google Map Maker team.