An isostatic gravity correction äg _AH giving the amount by which calculated gravity must be increased because the base of the crust extends below its average depth into the denser mantle in some regions (such as under mountainous regions) or lies considerably above it and is replaced by mantle in other regions (as under oceans). The isostatic theory of Airy and Heiskanen has the following postulates for use in computations: (a) the density of the crust is a constant 2670 kg/m3; (b) the density of the oceans is a constant 1030 kg/m3; (c) the density of the mantle is a constant 3270 kg/m3. The Airy Heiskanen gravity correction is the difference between the acceleration caused by attraction of matter extending from the bottom of the crust down to the depth of compensation and having the same density as the crust, and the acceleration caused by the attraction of matter actually in the same region and having a density 3270 kg/m3. It is usually calculated by taking the sum of the effects of cylindrical shells, or sectors patterned according to the Hayford zones, and extending from the average depth of the base of the crust to the actual location of the base. For distances out to 167. 7 km from a point, a cylinder of inner radius r1 and outer radius r2 and cut into n equal, vertical prisms contributes the amount äg _AH(force per unit mass or acceleration, depending on units used): äg _AH = (2ð G Äñ/n)((r₁² + D₁²)1/2 (r₂² + D₁²)½ (r₁² +D₂²)1/2 + (r₂² + D₂²)1/2), in which D₁ and D₂ denote D + d and D, respectively, if the prism is not in an oceanic region and denote D and D d', respectively, if the prism is in an oceanic region. In oceanic regions, the compensating layer begins at depth D d' and ends at depth D; in other regions, it begins at depth D and extends to depth D + d. G is the gravitational constant, Äñ is the difference between the density of the crustal material and the density of the liquid or plastic material supporting the crust. Heiskanen used values of 20, 30, 40, 60, 80 and 100 km for D. The formula assumes that the reference surface is flat. This assumption is not satisfactory if the effect of prisms more than 167. 7 km from the point of interest must be considered. This effect must be calculated using a spherical or ellipsoidal reference surface.