Abstract
The definition of the shape of the geoid is a fundamental objective of geodesy,
since it allows for the conversion between orthometric and ellipsoidal height systems.
The geoid can be computed from gravity values measured over the surface of the earth,
and considerable effort continues to achieve a global coverage of gravity values.
One technique that has been very successful in recent years in providing gravity
coverage in areas which previously have been too difficult to access is airborne
gravimetry. This technique has proved very useful in covering near offshore regions, for
example. The coastal regions of Australia are recognised as locations where airborne
gravimetry has the potential to fill in missing gravity data. A pilot survey using an
airborne gravity meter was undertaken off the north east coast of Australia.
In areas that remain unsurveyed it is sometimes useful to fill in the missing
gravity data values with predicted gravity values. Previous research has examined the
possibility of predicting gravity values from other observed quantities. The best success
has been achieved by using the gravity effect calculated from bathymetric information.
Often the corresponding isostatic compensation is computed, and the combined
bathymetric-isostatic gravity effect is used. However, the type and extent of
compensation that exists in any particular region mostly remains unknown. Theoretical
considerations indicate that the short wavelength part of the gravity field may be
adequately modelled by the gravity effect of the bathymetry alone, without reference to
an assumed compensation mechanism. With this in mind, a prediction scheme has been
developed which utilises the short wavelength gravity field information implied by the
bathymetry, combined with the long wavelength gravity field information from existing
observed gravity. This scheme allows the prediction of fill-in gravity values in areas
with limited observed gravity.
The prediction technique was used on a test set of data off the east coast of
Greenland. The prediction technique was seen to outperform a simple interpolation of
gravity values by approximately ten percent. Geoid computations performed with the
predicted gravity values indicate that the prediction technique can provide significant
improvements in computed geoids.