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Improved Geoid Modeling Using Observed and Modeled Gravity Gradients in Taiwan
(2017-05-01)
Hsiao, Yu-Shen
;
Hwang, Cheinway
;
Wu, Meng-Ling
;
Chang, Jung-Chieh
;
土木工程學系
The authors present a new geoid modeling procedure that can greatly improve relative geoid accuracy in mountainous areas, leading to improved applications for modern geodetic techniques, such as light detection and ranging (LIDAR), in mapping orthometric heights over steep terrain on which precise slopes are needed to assess the risk of landslides and the suitability of industrial development. The new procedure (1) measures gravity gradients or computing modeled gravity gradients from a regular grid of gravity anomalies, (2) uses these gradients to refine gravity anomalies, and (3) uses the gravity anomalies to compute geoidal undulations. This new procedure was tested in Taiwan. In situ gravity gradients were measured at approximately 4,000 gravity sites to compare the modeled gravity gradients. In the test, ground gravity observations are reduced to gravity anomalies at mean sea surface using three types of gravity gradients: normal, modeled, and observed. The researchers' geoid modeling uses the method of least-squares collocation (LSC) with the remove-compute-restore (RCR) procedure. Free-air gravity anomalies, as reduced using the observed and modeled gravity gradients, deviate from those using normal gradients by up to 100 mgal in high mountains. Using free-air gravity anomalies derived from observed and modeled gravity gradients, the authors can improve the relative geoid accuracies by up to 17 and 18cm, respectively, for Route 3 (an area with high mountains) in Taiwan. (C) 2016 American Society of Civil Engineers.
High-resolution depth and coastline over major atolls of South China Sea from satellite altimetry and imagery
(2016-04)
Hsiao, Yu-Shen
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Hwang, Cheinway
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Cheng, Yung-Sheng
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Chen, Liang-Chien
;
Hsu, Hung-Jui
Current bathymetric models for the South China Sea (SCS) are largely based on predicted depths from gravity and sparse single-beam echo-sounder measurements. Such models lack high-resolution coastlines and shallow water bottom features around atolls and islands. This study refines the gravity field of the SCS using sea surface heights from measurements of satellite altimeter Geosat/GM, ERS-1/GM, Jason-1/GM and the original Cryosat-2. A new one-minute gravity anomaly grid is determined. The modeled gravity anomalies show a 6-mgal RMS discrepancy with shipborne measurements in shallow waters. An altimeter-only bathymetric model is derived from the new gravity grid by the gravity-geological method that uses the latest global and regional models of the ocean depth and marine gravity as a priori knowledge. The new model outperforms current SCS bathymetric models and is accurate to 100 m, based on comparison with multi-beam depth measurements. Optical images from IKONOS-2, QuickBird-2, GeoEye-1, WorldView-1-2 and -3, are rectified and digitized to derive the zero (coastline) and 20-m depth contours (reef lines) around 44 atolls, which are integrated with the altimeter-only depths, giving significantly improved accuracies and spatial resolutions in modeled depths. The improvement percentages of coastlines by the satellite imagery range from 50% to 97% at 41 of the 44 atolls. We establish a webpage for free access to the optical and depth images, and the depth and gravity grids. We will continue to update satellite images, altimeter-derived gravity grids and bathymetric models over major atolls of the SCS. (C) 2016 Elsevier Inc. All rights reserved.
Topography-Assisted downward Continuation of Airborne Gravity: An Application for Geoid Determination in Taiwan
(2010-08-01)
Hsiao, Yu-Shen
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Hwang, Cheinway
;
土木工程學系
;
Department of Civil Engineering
We investigated the Fourier transform-based downward continuation (DWC) of airborne gravity anomalies around Taiwan assisted by topographic information. The topographic data are from the latest collections of elevations and ocean depths. The DWC employs a remove-compute-restore (RCR) procedure in which the topography is removed prior to computation and then restored to achieve stable solutions. The topographic gravity effect is evaluated point-wise using the Gaussian quadrature. A Gaussian filter with an optimal smoothing parameter reduces the noise-amplifying effect of DWC. Use of topography in DWC leads to improvements of 3 to 6 mgal of gravity on land. Surface and downward continued gravity anomalies are used to determine geoidal heights by least squares collocation (LSC) in a similar RCR procedure through the same topographic data. The accuracy of the geoidal heights at Taiwan's first-order leveling benchmarks is improved by 1 to 2 cm due to inclusion of downward continued airborne gravity.
Gravity anomalies from retracked ERS and Geosat altimetry over the Great Lakes: Accuracy assessment and problems
(2008-04-01)
Kingdon, Robert
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Hwang, Cheinway
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Hsiao, Yu-Shen
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Santos, Marcelo
;
土木工程學系
Lake waters are a useful area for investigating the response of the Earth's gravity field to topographical density variations, since lake beds represent large density contrasts between water and rock. However, our ability to investigate relations between lake bathymetry and gravity field effects is limited because of relatively sparse gravity data coverage over most lake surfaces, and the inability of satellite-derived gravity fields to capture the high frequency response to density effects. Satellite altimetry may provide an alternate method to determine a dense gravity field at surfaces of large inland lakes. We calculate satellite altimetry derived free-air (FA) anomalies over the Great Lakes in Canada, and evaluate them in terms of resolution and accuracy. The anomalies are compared to those calculated from the GRACE-derived EIGEN-GL04C gravity field to test for bias, and from shipborne or airborne results (where available) to test the accuracy of their determination of the high frequency component of the gravity field. Our results show that accuracies may reach less than 10 mgal, both in terms of bias and higher frequency effects, but are often tens of milligals. Furthermore, we find that altimetry does provide a higher resolution gravity field than satellite-derived gravity fields, and so if the accuracy can be improved in later efforts, it will be a useful tool for investigations of topographical density effects of lake waters.
Zonal and meridional ocean currents at TOPEX/Poseidon and JASON-1 crossovers around Taiwan: Error analysis and limitation
(2008-04-01)
Hwang, Cheinway
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Shih, Hsuan-Chang
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Guo, Jinyun
;
Hsiao, Yu-Shen
;
土木工程學系
A crossover method for determining zonal and meridional ocean current components is examined using data at three crossovers of TOPEX/Poseidon and JASON- I ground tracks over 2002 - 2006. To implement this method, a geoid model around Taiwan is constructed using surface and airborne gravity data. The modeled and observed geoidal heights at coastal benchmarks are consistent to 5 cm RMS with the means removed. The error and limitation of this method are discussed, concluding that, in order to obtain current velocities at a 10 cm s(-1) accuracy and a 6-km resolution, the dynamic ocean topography (DOT) at a mm-level accuracy is needed, which is not possible to achieve today. By filtering DOT to a spatial scale of 100 km or coarser, a 10 cm s(-1) accuracy of velocity may be obtained. One crossover (A) is situated south of Taiwan and near the Kuroshio, the second (B) is at the axis of the Kuroshio and the third is located in the northern Taiwan Strait. These three crossovers feature different ocean current patterns. At a spatial scale of 120 km, the agreement among the altimeter, the Princeton Ocean Model (POM), and the drifter-derived velocities is the best at B, followed by that at A, and then C. In fact, at C the altimeter-derived velocities contradict the POM-derived values, and the tide model error is to be blamed. Further improvement on geoid modeling is suggested.
Bathymetry enhancement by altimetry-derived gravity anomalies in the East Sea (Sea of Japan)
(2010-12-01)
Kim, Kwang Bae
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Hsiao, Yu-Shen
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Kim, Jeong Woo
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Lee, Bang Yong
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Kwon, Yi Kyun
The gravity-geologic method (GGM) was used to enhance the bathymetry of the East Sea (Sea of Japan) with satellite altimetry-derived free-air gravity anomalies and shipborne depth measurements. By comparison with the bathymetry model of Smith and Sandwell's (SAS) approach (1994), GGM was found to have an advantage with short wavelength (<= 12 km) components, while SAS better predicts longer wavelength (>= 25 km) components, despite its dependency on density contrast. To mitigate this limitation, a tuning density contrast of 10.25 g/cm(3) between seawater and the seafloor was primarily estimated by the downward continuation method and then validated by the check points method with GGM. Similarly, SAS is limited by the "A" value in low-pass part of the Wiener filter, which defines the effective range of the wavelength components on bathymetry. As a final result, we present an enhanced GGM bathymetry model by integrating all available data.
New free-air and Bouguer gravity fields of Taiwan from multiple platforms and sensors
(2014-01-25)
Hwang, Cheinway
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Hsu, Hung-Jui
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Chang, Emmy T. Y.
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Featherstone, W. E.
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Tenzer, Robert
We construct 1' x 1' grids of free-air and Bouguer gravity anomalies around Taiwan with well-defined error estimates for quality assessment. The grids are compiled from land, airborne and shipbome gravity measurements, augmented with altimeter gravity at sea. Three sets of relative land gravity measurements are network-adjusted and outlier-edited, yielding accuracies of 0.03-0.09 mGal. Three airborne gravity sets are collected at altitudes 5156 and 1620 m with accuracies of 2.57-2.79 mGal. Seven offshore shipborne gravity campaigns around Taiwan and its offshore islands yield shallow-water gravity values with 0.88-2.35 mGal accuracies. All data points are registered with GPS-derived geodetic coordinates at cm-dm accuracies, allowing for precise gravity reductions and computing gravity disturbances. The various datasets are combined by the band-limited least-squares collocation in a one-step procedure. In the eastern mountainous (or offshore) region, Bouguer anomalies and density contrasts without considering the oceanic (or land) topographic contribution are underestimated. The new grids show unprecedented tectonic features that can revise earlier results, and can be used in a broad range of applications. (C) 2013 Elsevier B.V. All rights reserved.
Analysis of Alabama Airborne Gravity at Three Altitudes: Expected Accuracy and Spatial Resolution from a Future Tibetan Airborne Gravity Survey
(2013-08-01)
Huang, Chi-Hsun
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Hwang, Cheinway
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Hsiao, Yu-Shen
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Wang, Yan Ming
;
Roman, Daniel R.
In situ airborne gravity data at altitudes of 11, 6.3, and 1.7 km over a smooth area of Alabama are used to assess gravity accuracy and errors in upward and downward continuations. Analysis of the Alabama free-air anomaly gravity data at crossover points at the three altitudes suggests 1 - 2 mgal accuracy for the dataset. Gravity data at each altitude are then expanded into local 3D Fourier series, to prepare for continuation. This Fourier representation results in continuation errors at few-mgal level in Alabama, even in the extreme case of downward continuation from 11 km to sea level. The result in Alabama inspires an airborne gravity survey over the rough, inaccessible terrain of Tibet. Similar investigations as in Alabama are made in Tibet using EGM08-derived airborne gravity data at flight altitudes of 10, 5, and 0 km. Bouguer anomalies at the 10-km altitude preserve the major tectonic features of Tibet. Downward continuation errors increase with terrain roughness, but the survey can enhance local tectonic features. This study highlights the value of a future Tibetan airborne gravity survey and points out the expected gravity accuracy and spatial resolution from this survey.
Bathymetry Estimation Using the Gravity-Geologic Method: An Investigation of Density Contrast Predicted by the Downward Continuation Method
(2011-06-01)
Hsiao, Yu-Shen
;
Kim, Jeong Woo
;
Kim, Kwang Bae
;
Lee, Bang Yong
;
Hwang, Cheinway
The downward continuation (DWC) method was used to determine the density contrast between the seawater and the ocean bottom topographic mass to estimate accurate bathymetry using the gravity-geologic method (GGM) in two study areas, which are located south of Greenland (Test Area #1: 40 - 50 degrees W and 50 - 60 degrees N) and south of Alaska (Test Area #2: 140 - 150 degrees W and 45 - 55 degrees N). The data used in this study include altimetry-derived gravity anomalies, shipborne depths and gravity anomalies. Density contrasts of 1.47 and 1.30 g cm(-1) were estimated by DWC for the two test areas. The considerations of predicted density contrasts can enhance the accuracy of 3 similar to 4 m for GGM. The GGM model provided results closer to the NGDC (National Geophysical Data Center) model than the ETOPO1 (Earth topographical database I) model. The differences along the shipborne tracks between the GGM and NGDC models for Test Areas #1 and #2 were 35.8 and 50.4 m in standard deviation, respectively. Furthermore, these differences were more strongly correlated with gravity anomalies than bathymetry in the test areas. It is shown that an accuracy of under 40 m can be obtained with comparisons to shipborne depths only in Test Area #1.
Airborne Gravity Surveys Over Taiwan Island and Strait, Kuroshio Current and South China Sea: Comparison of GPS and Gravity Accuracies at Different Flight Altitudes
(2012)
Hwang, Cheinway
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Shih, Hsuan-Chang
;
Hsiao, Yu-Shen
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Huang, Chi-Hsun
;
土木工程學系
We compare the accuracies of GPS and gravity observations obtained from three airborne gravity surveys over Taiwan Island at altitude 5000 m and over Kuroshio Current, Taiwan Strait, and Dongsha Atoll at altitude 1500 m. A kinematic network adjustment was used to determine the positions of the aircrafts. GPS positioning errors are at the decimeter-level, which are not entirely propagated to velocity and acceleration errors due to cancellation of long wavelength errors. Outliers are downweighted in the Gaussian filtering to improve the gravity accuracy, especially at altitude 1500 m. Compared with the upward-continued gravity, the gravity anomalies from the 1500-m surveys show a consistent accuracy of about 3 mgal; the accuracy from the 5000-m survey is degraded, especially over high mountains. The RMS crossover differences at 1500 m and 5000 m are all below 3 mgal, suggesting flight altitudes do not affect the crossover difference. Coherence analysis suggests that the spatial resolvable wavelengths of the airborne gravity range from 4 km (altitude 1500 m) to 6 km (altitude 5000 m).

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