Geodetic use of global digital terrain and crustal databases in gravity field modeling and interpretation

The release of global digital databases for the description of the Earth's topography and the shape of the Earth's crust in terms of consistency and geometry initiates a new era in the interpretation and analysis of the observed gravity field of our planet. The permanent increase in resolution of these databases permits furthermore the identification of high frequency gravity field components, a feature that is of special interest in applications of local or regional scales. The derivation of topographic/isostatic gravity models is the tool which reveals the gravity content of terrain and crustal databases in the spectral domain. We review the significance of some current global digital models in the frame of this analysis by computing distinct spectral gravity quantities and compare them against the Kaula rule of the gravity signal decay and the recently released reference gravity model EGM2008. The different isostatic hypothesis that can be applied in the derivation of a topographic/isostatic model as well its dependency with the increasing harmonic degree is demonstrated and quantified in terms of geoid heights and gravity anomalies. It is shown that the two fundamental compensation mechanisms, namely Airy and Pratt, act complementary in terms of their compensation effect to the uncompensated topography spectrum. The Airy mechanism reduces the uncompensated topography in the longer and medium wavelength part of the spectrum (up to degree 400), while Pratt acts in a compensating manner only for the high to very high frequencies, from degree 100 and onwards.