New Zealand gravity map series

Abstract

The principal gravity map series in New Zealand is at a scale of 1 : 250,000, and is based on gravity surveys designed to cover the entire country at an optimum station density of 1 per 10 km². Gravity anomalies are calculated on a uniform basis : they comprise the free-air anomaly, using the International Gravity Formula 1930, the Bouguer anomaly for a standard crustal density of 2·67 Mg/m³ with topographic correction to Hayford zone 0 (166·7 km), and isostatic anomalies on the Airy-Heiskanen system (including indirect effect), with normal thickness of crust T = 30 km. The accuracy of anomalies is mostly in the range from 1 to 2 mgal, though in mountainous areas it may reach 5 to 10 mgal. Gravity anomaly maps at 1: 250,000 are being issued in both Bouguer and isostatic anomaly versions, and maps of isostatic vertical gradient anomalies are also being issued, having been derived by numerical differentiation from the observed gravity anomalies. Maps of Bouguer and isostatic anomalies have also been published on a scale to 1 : 4,000,000.