ground truth
 n.— «Weather spotters provide what’s called “ground truth” to the National Weather Service and emergency weather management. “We’re needed because cloud-piercing Doppler radar is very good at helping the National Weather Service see what’s going on in the upper atmosphere, but it’s unable to detect what’s actually happening on the ground because of the curvature of the Earth.”» —“Volunteers track storms” by Debra Carr-Elsing Capital Times (Madison, Wisconsin) Oct. 26, 2006. (source: Double-Tongued Dictionary)

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