For a landmass that is isostatically compensated (its topographic mass excess balanced by a low-density root at depth, as in Airy isostasy), the Bouguer gravity anomaly is not zero (unlike the free-air anomaly, which tends toward zero over compensated topography); instead, the Bouguer anomaly becomes systematically negative over elevated, compensated terrain because the Bouguer correction removes the attraction of the visible topographic slab while the compensating low-density root at depth continues to produce a negative gravity contribution, well approximated for a simple slab model by the relation Bouguer anomaly is approximately equal to -2 x pi x G x rho x h.