The value H^B_N found for the orthometric elevation H _N of a point P_N by dividing the value of the potential function W _N per unit mass at P_N (considering the value of the potential function on the geoid to be 0) by an approximation 1/2(t_f + g _N) to the average value of the acceleration of gravity between the geoid and P_N. g _N is the value of the acceleration at P_N, t_f is the value of the acceleration (calculated by a standard gravity formula) on the geoid at the latitude f of P_N: H^B_N = 2W _N/t_f + g _N). W _N is calculated from the measured differences DH _n of elevation between points P_n-1 and P_n along a route from point P_o on the geoid to P_N, and the measured or otherwise determine values gn of the acceleration at P_n: W _N = S g _N DH _n, (n = 1 to N) or, for infinitesimal separations dH between points along the route,