AbstractFormulae to calculate the gravity effect due to 1°averaged topographic heights or depths on the idealised earth, are duly modified to apply to the reduction of corresponding topographic-isostatic effects on the Airy-Heiskanen hypothesis for any normal thickness Toof the earth's crust.Topographic-isostatic effects are calculated using electronic computers, for the entire earth, that is, for the Hayford zones A to I at the mean positions of l°×1°squares of latitudes and longitudes and then added to the given l°×1°mean free-air gravity anomaly-values computed at the identical positions in order to obtain the required Airy isostatic gravity anomalies for any normal thickness of the earth's crust.