AUSGeoid98 Computational Techniques

AUSGeoid98 uses the latest computational techniques and software available in Australia. A brief description of these techniques is given below. A more complete description can be found either in one of the AUSGeoid98 technical papers (eg Johnston and Featherstone, 1998 [PDF_203k]), or in the references supplied below.

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Technique descriptions

• Gravity observations are typically aliased by the terrain in the area of survey. Where the terrain is rugged, observations are often taken alongside roads or tracks. In areas of low terrain variation, like some parts of central Australia, observations were often taken on hilltops where helicopters could be landed. To account for these aliasing effects, a reconstruction technique has been used, where the GEODATA nine second DEM has been used to reconstruct the gravity anomalies to give a more representative mean free-air gravity anomalies.
• The gravimetric terrain correction (Moritz, 1968) is applied to the raw gravity data to account mathematically for the approximations used in Stokes's formula.
• The primary and secondary indirect effect corrections (Wichiencharoen, 1982) are added to the computed geoid. These corrections complement the gravimetric terrain correction. Both these corrections are highly dependant on topography, and as such are largest in areas of rugged terrain.
• A "draping" technique (Kirby and Forsberg, 1997), which is based on a Least Squares Collocation algorithm, is used to adjust the satellite-altimeter-derived gravity anomalies to the AGSO (now Geoscience Australia) marine gravity tracks offshore. This was found necessary to account for loss of satellite altimeter lock and near-shore sea surface topography. It also allows for a smooth transition from the near coastal land gravity observations to the marine observations. This approach removes the coastal edge effects seen in AUSGeoid93.
• The remove-compute-restore technique has been utilized, where the EGM96 global geopotential model has been used as base. This technique is a standard in modern geoid computations and was also used in AUSGeoid93.
• A one-dimensional Fast Fourier Transform (FFT) technique (Haagmans et al 1993) was used to perform a Stokesian integration of the Faye (terrain corrected free-air) gravity anomalies to calculate the residual geoid-EGM96 separations.
• To overcome the approximation caused by only using Australian data in the computation of AUSGeoid98, a modification to Stokes's integration kernel (Featherstone et al 1998) has been used. This modification makes a Least Squares minimisation of the error associated with neglecting global data and causes the Fourier series of this error to converge quickly to zero.
• A one-degree cap radius has been used. Tests have shown that the geoid fit to the Australian Height Datum (AHD) is best at this cap size.

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References

Featherstone, W.E., Evans, J.D., Olliver, J.G., (1998). A Meissl-modified Vanicek and Kleusberg kernel to reduce truncation error in gravimetric geoid computations. Journal of Geodesy, 72:154-160.

Haagmans, R., de Min, E van Gelderen, M., (1993). Fast evaluation of convolution integrals on the sphere using 1D-FFT, and a comparison with existing methods for Stokes's integral. Manuscripta Geodaetica, 18(5): 227-241.

Kirby, J.F., and Forsberg, R., (1998). A comparison of techniques for the integration of satellite altimeter and surface gravity data for geoid determination. in :Forsberg, Feissl, Deitrich(eds), Geodesy on the move, Springer, Berlin.

Moritz, H., (1968). On the use of the terrain correction in solving Molodensky's problem, Report 108, Department of Geodetic Science and Surveying, Ohio State University.

Wichiencharoen, C., (1982). The indirect effects on the computation on geoid undulations, Report 336, Department of Geodetic Science and Surveying, Ohio State University.

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