Secular Geoid Rate from GRACE for Vertical Datum Modernization
GRACE-derived geoid rates are studied for North-America, where the adjustment of the Earth to ancient ice sheets causes a secular geoid increase up to 1.3 mm/year. These significant geoid changes are of particular interest for establishing a new geoid-bas
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Secular Geoid Rate from GRACE for Vertical Datum Modernization W. van der Wal, E. Rangelova, M.G. Sideris, and P. Wu
Abstract GRACE-derived geoid rates are studied for North-America, where the adjustment of the Earth to ancient ice sheets causes a secular geoid increase up to 1.3 mm/year. These significant geoid changes are of particular interest for establishing a new geoidbased vertical datum in Canada and other high accuracy applications. To quantify the uncertainty of the derived rate of change of the geoid, several methods for GRACE error approximation are studied using: (i) calibrated standard deviations, (ii) a full covariance matrix, and (iii) residuals of least-squares fit of a trend and periodic variations to the time series of spectral coefficients. It is found that the residuals give the largest error estimates, probably because correlated errors are captured better. Furthermore, through maximizing the signal-to-noise ratio, it is found that the Swenson and Wahr (2006) filter of correlated GRACE errors should be applied to coefficients above degree 22 and order 4. Measurement errors are largely longitude independent, with magnitude around 0.06 mm/year. The largest geoid rate uncertainty is estimated in the area of present-day ice melt in Alaska and south of the Great Lakes and south-west of Hudson Bay (over 0.3 mm/year) due to uncertainty in continental water storage. For the creation of a geoid rate model based on GRACE data it is important that efforts are focused on reducing uncertainty in these areas, rather than improving post-processing.
W. Van derWal () Department of Geomatics Engineering, University of Calgary, Calgary, AB, T2N 1N4, Canada e-mail: [email protected]
81.1 Introduction The static geoid has reached an accuracy level where time-dependent effects on the geoid become significant. For applications where high geoid accuracy is needed, such as precise georeferencing, oceanography, hazard assessment and monitoring, a static geoid model that is provided by the national survey agency can be accompanied by a model of the secular changes of the geoid (or “dynamical vertical datum”). The GRACE satellite mission provides monthly gravity field solutions from which a secular geoid rate can be estimated. A dynamical vertical datum can be constructed by combining terrestrial and satellite data (Rangelova, 2007). However, in this paper we investigate measurement and systematic errors in the geoid rate from GRACE data alone. It should be mentioned that recently the Swedish national survey agency has incorporated a hybrid of terrestrial data and geophysical model for Glacial Isostatic Adjustment (GIA) to homogenize leveling observations (Ågren and Svensson, 2007) for readjustment of the leveling network. The study area is North America, where the dominant source of long-term geoid change is GIA. This process is an ongoing response to melting of ice sheets that covered a large part of North America roughly 20,000 years ago. The rebound of the crust is accompanied by mass inflow in the E
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