# Dictionary:Free-air correction

**1**. A correction for the elevation of a gravity measurement required because the measurement was made at a different distance from the center of the Earth than the datum. The correction is 0.308 768–0.000 440sin^{2}λ–0.000 000 1442*h* mGal/m, where λ is the latitude and *h* is elevation above the ellipsoid. Usually only the first term is used, 0.3086 mGal/m or 0.09406 mGal/ft. See Figure G-7. **2**. In Turam, normalizing a ratio of successive measurements by dividing by the calculated free-space vertical magnetic field. Compare *normal correction*.