Editing Free-air gravity anomaly (section)

== Survey methods ==
The free-air correction adjusts measurements of gravity to what would have been measured at mean sea level, that is, on the geoid. The gravitational attraction of Earth below the measurement point and above mean sea level is ignored and it is imagined that the observed gravity is measured in air, hence the name. The theoretical gravity value at a location is computed by representing the Earth as an [[ellipsoid]] that approximates the more complex shape of the geoid. Gravity is computed on the ellipsoid surface using the [[Theoretical gravity|International Gravity Formula]]. 

For studies of subsurface structure, the free-air anomaly is further adjusted by a correction for the mass below the measurement point and above the reference of mean sea level or a local datum elevation.<ref name="Telford1990" /> This defines the [[Bouguer anomaly]].