The accuracy of the global solution is of the order of three meters in geoid height or 9 milligals in gravity anomalies. These estimates are based on the formal statistics computed for the solution but are in excellent agreement with tests made with in dependent surface gravity data and with astro- geoid data. Figures 2 and 3 give some typical examp les and more information is given in [1, 2, 3 and 6]. The interpretation of the earth's gravitational field In this discussion we will limit ourselves to the im plications of the long wavelength features in the earth's gravity field rather than the small features which are the consequence of near surface density anomalies and of the topography. By long wave length we mean gravity anomalies that have an areal extent of about 1000 km diameter and, as discussed below, these anomalies will be caused by density anomalies at some depth below the earth's surface. If the earth acted essentially like a fluid over a suitably long time span, it would take on a spheroi dal shape which could be calculated from the pre- cessional constant and its angular velocity [7], Such a body is said to be in hydrostatic equili brium and all level surfaces inside the body would be at the same time surfaces of equal density. For a body of the earth's mass and the dynamical ellipticity calculated from the precession constant, the geometric flattening of an ellipsoid correspond ing to the hydrostatic figure would be about 1/300. Figure 4 shows the gravity field as it departs from such a hydrostatic figure and the anomalies clearly indicate that the earth differs considerably from a rotating liquid. This is hardly surprising as we are quite used to a more or less rigid earth outside of the oceans. But we emphasize that we are talking about the shape of the earth over a long time span so that when materials are subjected to large forces over a long time span we can expect them to re adjust in one way or another in response to these forces. That this happens in the case of the earth, to at least some extent, is evidenced by the isostatic compensation of mountain ranges and by areas of post glacial rebound such as Fenno-Scandinavia. Two alternative interpretations are possible as to what supports these departures from hydrostatic equilibrium. The first is a finite strength interpreta tion whereby the earth materials are assumed to be sufficiently rigid to support over very long time periods the stresses and strains caused by the gravity anomalies. The second interpretation is that the earth's strength is low but that the density anoma lies are dynamically maintained by some form of thermal convection. Seismology gives us some in dication as to which of these two interpretations is most likely and in fact it appears that the two co-exist. Seismology also indicates that the core of the earth is sufficiently liquid so as not to depart significantly from hydrostatic equilibrium so that we need only consider the earth's mantle and crust in the following discussion. The first characteristic of the gravity field is the difference between the hydrostatic flattening of about 1/300 and the observed flattening of 1 /29S.255. The difference may appear small but is real and is considerably larger than the uncertainties associated with either flattening estimate. The difference im plies that the earth's bulge is too large by some 0.5%, and the question is whether this departure is sufficiently significant, when compared to the other 5° x 5° AVERAGES COMBINATION SOLUTION 20 S -20 E 80° 60° LONGITUOE Fig. 3 Comparisons of gravity profiles based on the global solution (solid lines) and on data collected by surface measurements (broken lines). Indian Ocean. 0 46 ngt 72

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Nederlands Geodetisch Tijdschrift (NGT) | 1972 | | pagina 8