A gravity anomaly is the difference between a prediction of gravity and the actual observed gravity - they suggest hidden variations in the subsurface, like caves or mineral deposits.
In order to understand the anomaly a number of corrections need to be made to the measurement of gravity.

1. The theoretical gravity should be removed in order to only leave local effects.
2. The elevation of the point where each gravity measurement was taken must also be taken into account, as altitude effects the measurement of the gravity of the Earth itself. The free-air correction does this by adjusting the measurements of gravity to what they would have been if they had been measured at sea level..

When the Free-air correction is combined with the removal of theoretical gravity, this leaves leaves the free-air anomaly .