The topographic bias in gravimetric geoid determination revisited

The topographic potential bias at geoid level is the error of the analytically continued geopotential from or above the Earth’s surface to the geoid. We show that the topographic potential can be expressed as the sum of two Bouguer shell components, where the density distribution of one is spherical symmetric and the other is harmonic at any point along the normal to a sphere through the computation point. As a harmonic potential does not affect the bias, the resulting topographic bias is that of the first component, i.e. the spherical symmetric Bouguer shell. This implies that the so-called terrain potential is not likely to contribute significantly to the bias. We present three examples of the geoid bias for different topographic density distributions.

Analytical computation of the full gravity tensor of a homogeneous arbitrarily shaped polyhedral source using line integrals

The analytical computation of the full gravity tensor from a polyhedral source of homogeneous density is presented, with emphasis on its algorithmic implementation. The theoretical development is based on the subsequent transition of the general expressions from volume to surface and from surface to line integrals, defined along the closed polygons building each polyhedral face. However, the accurate numerical computation of the obtained transcendental expressions is linked with the relative position of the computation point and its corresponding projections on the plane of each face and on the line of each segment with respect to the polygons defining each face. Depending on this geometric setup, the application of the divergence theorem of Gauss leads to the appearance of additional correction terms, valid only for these boundary conditions and crucial for the correct numerical evaluation of the polyhedral-related gravity quantities at those locations of the computation point. A program in Fortran is supplied and thoroughly documented; it computes the gravitational potential, its first-order derivatives, and the full gradiometric tensor at arbitrary space points due to a general polyhedral source of constant density.

Solving the Topographic Potential Bias as an Initial Value Problem

Solving the Topographic Potential Bias as an Initial Value ProblemIf the gravitational potential or the disturbing potential of the Earth be downward continued by harmonic continuation inside the Earth's topography, it will be biased, the bias being the difference between the downward continued fictitious, harmonic potential and the real potential inside the masses. We use initial value problem techniques to solve for the bias. First, the solution is derived for a constant topographic density, in which case the bias can be expressed by a very simple formula related with the topographic height above the computation point. Second, for an arbitrary density distribution the bias becomes an integral along the vertical from the computation point to the Earth's surface. No topographic masses, except those along the vertical through the computation point, affect the bias. (To be exact, only the direct and indirect effects of an arbitrarily small but finite volume of mass around the surface point along the radius must be considered.) This implies that the frequently computed terrain effect is not needed (except, possibly, for an arbitrarily small inner-zone around the computation point) for computing the geoid by the method of analytical continuation.