theodolite [key], calibrated optical instrument used to determine relative position in surveying, navigation, and meteorology. It is similar in construction to a surveyor's transit, consisting of a telescope fitted with a spirit level and mounted on a tripod so that it is free to rotate about its vertical and horizontal axes. (Sometimes two or more telescopes of different magnifications are used.) Graduated scales are used to measure the amount of rotation about the axes. Measurements of the altitude and azimuth of a weather balloon at precise time intervals are used to compute the estimated wind velocity of the atmosphere.
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