A special template (shown in Figure 2) made of transparent material is placed on the contour map, centered over the point where N is to be determined. Referring to the formula and the template, γ_m is the mean value of normal gravity, ψ is the angular distance between the point where N is to be determined (m1) and the area where the effect of Δg is being considered, α is the azimuth from the affected point (m1) to the causing effect (dm), and