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Abstract:A global geopotential model, like EGM2008, is not capable of representing the highfrequency components of Earth's gravity field. This is known as the omission error. In mountainous terrain, omission errors in EGM2008, even when expanded to degree 2190, may 2 reach amplitudes of 10 cm and more for height anomalies. The present paper proposes the utilisation of high-resolution residual terrain model (RTM) data for computing estimates of the omission error in rugged terrain. RTM elevations may be constructed as the difference between the SRTM (Shuttle Radar Topography Mission) elevation model and the spherical harmonic topographic expansion DTM2006.0. Numerical tests, carried out in the GermanAlps with a precise gravimetric quasigeoid model (GCG05) and GPS/levelling data as references, demonstrate that RTM-based omission error estimates improve EGM2008 height anomaly differences by 10 cm in many cases. The comparisons of EGM2008-only height anomalies and the GCG05 model showed 3.7 cm standard deviation after a bias-fit. Applying RTM omission error estimates to EGM2008 reduces the standard deviation to 1.9 cm which equates to a significant improvement rate of 47%. Using GPS/levelling data strongly corroborates these findings with improvement rates of 49% . The proposed RTM approach may be of practical value to improve quasigeoid determination in mountainous areas without sufficient regional gravity data coverage, e.g., in parts of Asia, South America or Africa. As a further application, RTM omission error estimates will allow a refined validation of global gravity field models like EGM2008 from GPS/levelling data.

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We assess the new EGM2008 Earth gravitational model using a set of 1056 astrogeodetic vertical deflections over parts of continental Europe. Our astrogeodetic vertical deflection data set originates from zenith camera observations performed during 1983–2008. This set, which is completely independent from EGM2008, covers, e.g., Switzerland, Germany, Portugal and Greece, and samples a variety of topography – level terrain, medium elevated and rugged Alpine areas. We describe how EGM2008 is used to compute vertical deflections according to Helmert's (surface) definition. Particular attention is paid to estimating the EGM2008 signal omission error from residual terrain model (RTM) data. The RTM data is obtained from the Shuttle Radar Topography Mission (SRTM) elevation model and the DTM2006.0 high degree spherical harmonic reference surface. The comparisons between the astrogeodetic and EGM2008 vertical deflections show an agreement of about 3 arc seconds (root mean square, RMS). Adding omission error estimates from RTM to EGM2008 significantly reduces the discrepancies from the complete European set of astrogeodetic deflections to 1 arc second (RMS). Depending on the region, the RMS errors vary between 0.4 and 1.5 arc seconds. These values not only reflect EGM2008 commission errors, but also short‐scale mass‐density anomalies not modelled from the RTM data. Given (1) formally stated EGM2008 commission error estimates of about 0.6–0.8 arc seconds for vertical deflections, and (2) that short‐scale mass‐density anomalies may affect vertical deflections by about 1 arc second, the agreement between EGM2008 and our astrogeodetic deflection data set is very good. Further focus is placed on the investigation of the high‐degree spectral bands of EGM2008. As a general conclusion, EGM2008 – enhanced by RTM data – is capable of predicting Helmert vertical deflections at the 1 arc second accuracy level over Europe.

The Federal Office of Topography swisstopo is responsible for the maintenance of the coordinate reference frames in Switzerland. Beside the static reference frames, used for national surveying, the development of a kinematic model, mainly used for scientific investigations, is under development since many years. For