In case where the effects of terrain are removed precisely, we speak of a refined or complete Bouguer reduction. In case the terrain is approximated by a flat plate of thickness , the height of the gravity measurement location above sea level, we speak of a simple Bouguer reduction. The difference between the two, the differential gravitational effect of the unevenness of the terrain, is called the terrain effect. It is always negative.
The equation for the simple Bouguer reduction is δG_f = 2Hρπγ, where H is the thickness of the plate, γ is the constant of gravitation and ρ is the density of the material.