Quasi-Geoid of New Caledonia: Computation, Results and Analysis
The archipelago of New Caledonia is located in the Pacific Ocean, near the New Hebrides Trench. The gravity field is extremely rough in this area, free-air gravity anomalies varying from –230 to +200 mgal. Two vain attempts to compute a geoid model in 200
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Quasi-Geoid of New Caledonia: Computation, Results and Analysis P. Valty and H. Duquenne
Abstract The archipelago of New Caledonia is located in the Pacific Ocean, near the New Hebrides Trench. The gravity field is extremely rough in this area, free-air gravity anomalies varying from –230 to +200 mgal. Two vain attempts to compute a geoid model in 2000 and 2004 led the Territorial Government of New Caledonia to put IGN (French National Geographic Institute) in charge of a new calculation. Gravity data were provided by several institutions (Bureau Gravimétrique International, Institut de Recherches pour le Développement, Genavire, IGN) and were of various quality, so that data cleaning has been an important stage. Using the GRAVSOFT package, height anomalies have been computed by the residual terrain method combined with Stokes integration. By comparison of the quasi-geoid with levelled GPS points on the main island, accuracy has been estimated to be better than 9 cm, which shows a significant improvement with previous solutions. A grid to convert ellipsoidal heights into heights above mean sea level has been derived from the quasi-geoid and levelled GPS points. Its accuracy is about 4.5 cm. The shifts between vertical reference systems of principal islands of the archipelago have been determined. Differences between the geoid and the mean sea level at tide gauges, and their correlation with oceanic currents have also been studied and discussed in the paper.
P. Valty () IGN/LAREG, Champs sur Marne 77420, France e-mail: [email protected]
57.1 Introduction The archipelago of New Caledonia is located in the Pacific Ocean, near the New Hebrides Trench. The gravity field is extremely rough in this area, due to these particularly huge oceanic trenches. Free-air gravity anomalies are for example varying from –230 to +200 mgal (Fig. 57.1). The local organization DITTT (Direction des Infrastructures, de la Topographie et des Transports Territoriaux de la Nouvelle-Calédonie) charged first in 1999 the French ESGT (Ecole Supérieure des Géomètres et Topographes) to compute a first version of the quasi-geoid of New Caledonia. This computation (Duquenne, 2000) revealed gravimetric errors on the main island Grande-Terre. Through a new convention firmed in 2000 between the DITTT and the IGN (Institut Géographique National), it was decided to realize a new gravimetric campaign and a new version of the New-Caledonian quasigeoid. The gravity measurements were entrusted to the IGN’s SGN (Service de Géodésie et Nivellement) (IGN/SGN, 2003). During this time, the DITTT improved the quality of the GPS levelling points used for the determination of the grid of conversion between heights above the ellipsoid and altitudes. Thus, a new version of the quasi-geoid of NewCaledonia has been computed in 2004 by the LAREG (Laboratoire de Recherche en Géodésie, Marne-la-Vallée, France) (Charpentier, 2004). The accuracy of results has been really improved since 2000, but several gravimetric data still remained
S.P. Mertikas (ed.), Gravity, Geoi
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