In gravity measurements, the corrections applied to observed gravity values because of differences of station elevation to reduce them to an arbitrary reference or datum level, usually sea level. The corrections consist of (1) the free-air correction, to take care of the vertical decrease of gravity with increase of elevation, and (2) the Bouguer correction, to take care of the attraction of the material between the reference datum and that of the individual station. In seismic measurements, the corrections are applied to observed reflection time values due to differences of station elevation in order to reduce the observations to an arbitrary reference datum or fiducial plane.