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

Dinh Toan Vu,Georgios S Vergos,Sean Bruinsma,Sylvain Bonvalot,Dominique Remy

doi:10.3390/rs12050817

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.

Highlights

Global Navigation Satellite System (GNSS) technology is used for positioning and navigation activities because of its speed, convenience and accuracy
A high-resolution gravimetric-only quasigeoid model (GEOID_LSC) was recently determined for Vietnam [3]. This model has a mean bias of 50 cm, standard deviation (STD) of 9.7 cm and differences ranging from 13.6 to 81.6 cm when compared with a set of 812 GNSS/levelling points
The quasigeoid refers to a global reference system, i.e., an international reference gravity potential W0 = 636,853.4 m2s2, whereas the heights determined from levelling refer to the national Mean Sea Level (MSL), called the Vietnam Local Vertical Datum (VLVD)

Summary

Introduction

Global Navigation Satellite System (GNSS) technology is used for positioning and navigation activities because of its speed, convenience and accuracy. To convert an ellipsoidal height to a physical height, i.e., orthometric (H) and/or normal height (H*), one has to subtract the height anomaly (ζ), being the vertical distance between the quasigeoid and the reference ellipsoid, as follows: H* = h − ζ (1) This is the basic formula in GNSS levelling. A high-resolution gravimetric-only quasigeoid model (GEOID_LSC) was recently determined for Vietnam [3] This model has a mean bias of 50 cm, STD of 9.7 cm and differences ranging from 13.6 to 81.6 cm when compared with a set of 812 GNSS/levelling points. The quasigeoid refers to a global reference system, i.e., an international reference gravity potential W0 = 636,853.4 m2s2, whereas the heights determined from levelling refer to the national Mean Sea Level (MSL), called the Vietnam Local Vertical Datum (VLVD). The resulting standard deviations obtained for the most recent hybrid models range from a few cm up to 15 cm, depending on the quality of the available gravity and GNSS/levelling data for the hybrid geoid/quasigeoid determination

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Results

Conclusion