In geophysics, including gravimetry, the term Bouguer anomaly (or Bouguer gravity anomaly) is the gravity anomaly (of some point on a planet) after compensating for altitude and for the difference in mass underneath due to landforms (extra for a mountain, less, for a valley). The gravity anomaly still present if an adjustment is made only for altitude is termed the free-air anomaly, and the Bouguer anomaly additionally discounts the effect on gravity due to the mass of the landform. Thus, if you are on top of a mountain and measure a gravity anomaly, the free-air anomaly is the reduced anomaly after you have taken into account how far above sea level you are, and the Bouguer anomaly is what is left of the anomaly after you have taken into account the density and shape of the mountain beneath you. This remaining anomaly is that due to other density variations beneath you within the mountain or below it. The compensation for these two effects is termed the Bouguer correction or Bouguer reduction. The term Bouguer gravity refers to the gravity across the surface of the body after the Bouguer correction has been applied. A Bouguer gravity map is a map of the free-air gravity.
A simplified calculated Bouguer reduction, due essentially to the difference in nearby mass given the vertical difference in ground height, is termed the simple Bouguer reduction or incomplete Bouguer reduction, while one that takes into account more terrain detail is called the refined Bouguer reduction or complete Bouguer reduction.