A Quasigeoid-Derived Transformation Model Accounting for Land Subsidence in the Mekong Delta towards Height System Unification in Vietnam

Abstract

A vertical offset model for Vietnam and its surrounding areas was determined based on the differences between height anomalies derived from 779 Global Navigation Satellite System (GNSS)/levelling points and those derived from a dedicated high-resolution gravimetric-only quasigeoid model called GEOID_LSC. First, the deterministic transformation model to effectively fit the differences between the quasigeoid and GNSS/levelling heights was based on a third-order polynomial model. Second, the residual height anomalies have been interpolated to a grid employing Least-Squares Collocation. Finally, the distortions were restored to the residual grid. This model can be used for combination with a gravimetric quasigeoid model in GNSS levelling. The quality of GNSS/levelling data in Vietnam was analyzed and evaluated in this study. The annual subsidence rate from ALOS-1 was also used to analyze the effects of subsidence on the quality of GNSS/levelling data in the Mekong Delta. From this we made corrections to improve the accuracy of GNSS/levelling data in this region. The offset model was evaluated using cross-validation technique by comparing with GNSS/levelling data. Results indicate that the offset model has a standard deviation of 5.9 cm in the absolute sense. Based on this offset model, GNSS levelling can be carried out in most of Vietnam’s territory complying third-order levelling requirements, while the accuracy requirements for fourth-order levelling networks is met for the entire country. This model in combination with the developed gravimetric quasigeoid model should also contribute to the modernization of Vietnam’s height system. We also used high-quality GNSS/levelling data and the determined quasigeoid model to determine the geopotential value W0 for the Vietnam Local Vertical Datum. The gravity potential of the Vietnam Local Vertical Datum is estimated equal to W 0 LVD = 62,636,846.81 ± 0.70 m2s−2 with the global equipotential surface realized by the conventional value W0 = 62,636,853.4 m2s−2.

Determination of the geopotential value on the permanent GNSS stations in Vietnam based on the Geodetic Boundary Value Problem approach

“…This procedure is known as Height System Unification (HSU, Rapp & Balasubramania 1992;Ihde et al 2000;Grigoriadis et al 2014;Vergos et al 2018) and its products are the vertical datum parameters, which are the potential and/or the potential differences between local and global reference level. In Vu et al (2020), the zeroheight geopotential value was estimated for the LVD of Vietnam using a local gravimetric-only quasigeoid and height anomalies derived from GNSS/levelling data. However, tilts, caused by accumulated systematic levelling errors and land subsidence, are known to have affected the GNSS/levelling data in Vietnam.…”

“…In this study, we first perform direct determination of the zeroheight geopotential value of the LVD on the GNSS/levelling points using the GBVP approach to validate the proposed method as well as to show its advantages by comparing with the results derived from the GNSS/levelling data and gravimetric-only quasigeoid in Vu et al (2020). Then the GBVP approach will be used for determination of the geopotential values including variations in time on the GNSS-CORS stations.…”

“…Normal height is currently used in the national height system of Vietnam. In Vu et al (2020), assuming a normal distribution of the residuals, 33 observations were rejected. The 779 good quality points, which are relatively well distributed over the entire country as shown in Fig.…”

“…Its effect on the estimated disturbing potential for Vietnam is also determined. In Vu et al (2020), the datum offset (δ H * ) was estimated at about 69 cm with the global equipotential surface realised by the conventional value W 0 = 62 636 853.4 m 2 s -2 . We used this offset value in a simulation, in which terrestrial gravity anomalies are biased according to: g b = 0.3086 δ H * ≈ 0.2 (mGal).…”

“…It is evaluated on 779 GNSS/levelling points, for GOCE DIR-R5 at D/O 260, and after extending with EGM2008 from D/O 261-2190, as well as for the gravimetric-only quasigeoid GEOID LSC (Vu et al 2019). Table 2 shows the gravity potential offset value for the LVD with respect to W 0 = 62 636 853.4 m 2 s -2 calculated on 779 GNSS/levelling points (see Vu et al (2020) for details). The potential offset value computed with mixed DIR/EGM up to its full resolution is 6.64 m 2 s -2 and that computed with GEOID LSC is 6.68 m 2 s -2 , while the GOCE DIR-R5 only and mixed DIR/EGM up to D/O 719 yield 6.12 and 6.45 m 2 s -2 , respectively.…”