Implementing a Dynamic Geoid as a Vertical Datum for Orthometric Heights in Canada
The geoid heights in Canada are subject to secular dynamic changes caused by the slow glacial isostatic adjustment of the viscoelastic Earth. As a result, the reference surface for orthometric heights changes with time at a level that is an order of magni
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Implementing a Dynamic Geoid as a Vertical Datum for Orthometric Heights in Canada E. Rangelova, G. Fotopoulos, and M.G. Sideris
Abstract The geoid heights in Canada are subject to secular dynamic changes caused by the slow glacial isostatic adjustment of the viscoelastic Earth. As a result, the reference surface for orthometric heights changes with time at a level that is an order of magnitude smaller than the rate of change of heights. The objective of this paper is to provide a feasibility study on implementing the geoid as a dynamic vertical datum. For this purpose, the most accurate GPS ellipsoidal heights from the CBN (Canadian Base Network), orthometric heights from the most recent minimally constrained adjustment of the primary vertical control network and the latest geoid model for Canada are used. In this approach, the dynamic geoid is treated in the context of the combined adjustment of the ellipsoidal, orthometric and geoid heights. In this paper, it is shown that the present-day accuracy of the three height components precludes the implementation of the dynamic vertical datum, and the accuracy of the orthometric heights appears to be the limiting factor. By means of a simulated example, we demonstrate that the dynamic vertical datum requires an accuracy of 1.0–1.5 cm for each of the three height components. Provided this level of accuracy is reached, the vertical reference surface must be adjusted for the secular geodynamic effect after 8–10 years have elapsed from the reference epoch. For comparison, vertical crustal motion can cause significant systematic discrepancies among the ellipsoidal, orthometric, and geoid heights over a 2-year time interval.
E. Rangelova () Department of Geomatics Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada, T2N 1N4 e-mail: [email protected]
38.1 Introduction Canada’s official vertical datum (the reference surface for orthometric heights) is the Canadian Geodetic Vertical Datum of 1928 (CGVD28) constrained to the mean sea level of five tide gauges on the Pacific and Atlantic coasts (Véronneau, 2002). In addition to the large east-west tilt of the reference surface as a result of the imposed constraints, the datum is outdated, has limited coverage and large regional systematic errors of up to 1 m. Moreover, the glacial isostatic adjustment of the crust (e.g., Peltier, 2004) causes significant vertical crustal motion (uplift/subsidence) of benchmarks of the vertical control network. All of these effects led to the necessity for a new geoid-based vertical datum compatible with the GNSS positioning technique and easily accessible in the northern parts of Canada, where a vertical control network does not exist (Véronneau et al., 2006). The geoid in North America, however, experiences large secular rise (1–2 mm/year) as a result of the mass transport beneath the uplifting crust. These significant dynamic variations and the continuously improving accuracy of the regional geoid model (see Huang et al., 2006) aiming towards a cm-leve
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