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New gravimetric-only and hybrid geoid models of Taiwan for height modernisation, cross-island datum connection and airborne LiDAR mapping
(2020-08-12)
Hwang, Cheinway
;
Hsu, Hung-Jui
;
Featherstone, W. E.
;
Cheng, Ching-Chung
;
Yang, Ming
This paper combines gravity data collected from airborne, shipborne and terrestrial surveys and those derived from satellite altimetry to determine a high-resolution gravimetric and hybrid geoid model (on a 30" x 30 '' grid) in and around Taiwan. Some 6000 new land gravity values at a 0.03-mGal precision make a notable contribution to the geoid modeling. Shipborne gravity data in waters 20 km offshore Taiwan were collected to improve the coastal geoid precision. In a circular area of 50 km around each of the five major tide gauges in Taiwan, gravity data were measured to improve vertical datum connections between Taiwan and its four offshore islands. Height anomalies were computed first and then converted to geoid heights. At > 2000 benchmarks, we obtained measured geoid heights to assess the gravimetric-only geoid and to create a hybrid geoid. Our assessments and formal errors from least-squares collocation indicate few cm of standard deviations for both geoid models, but the gravimetric geoid has mean differences of up to 20 cm with the measured geoidal heights. The hybrid geoid is used in RTK-VBS orthometric heighting, achieving a 5-cm precision. The gravimetric geoid is used to determine the relative differences in the ocean's mean dynamic topography (MDT) between Taiwan and the four offshore islands, which are also compared with those from oceanic and altimetric methods for estimating MDT. Differences in MDT help to identify 41.7 cm and 54.1 cm offsets in the current vertical datums of Penghu and Lanyu islands. In a low-lying, flood-prone region of southern Taiwan, the hybrid geoid improves LiDAR mapping of sub-zero elevation zones by 20 cm, corresponding to 70 years of sea level rise at an assumed rate of 0.286 cm/yr.
Short-Time Geodetic Determination of Aquifer Storage Coefficient in Taiwan
(2018-12-01)
Chen, Kuan-Hung
;
Hwang, Cheinway
;
Chang, Liang-Cheng
;
Ke, Chien-Chung
;
土木工程學系
Specific yield (S-y) and specific storage (S-s) are two hydrogeological parameters often estimated in a costly pumping test (hydraulic method). Pumping or a natural aquifer recession can cause gravity change and surface deformation, useful for S-y and S-s determination by the more economic geodetic method. From 2013 to 2017, over a few days we measured gravity changes at four sites and subsidence at one site in Taiwan by a FG5 gravimeter and a precision level to estimate S-y and S-s. Using short-time gravity and level records avoids complicated logistic supports and temporal effects. The measured gravity changes are associated with the MODFLOW-modeled groundwater depletion. We succeeded in estimating S-y at two sites and failed at the other two. One successful case uses rapid postrain declines of gravity and groundwater level. We did leveling only at one site, but here the 500-min, continuous, submillimeter subsidence records result in a S-s consistent with the hydraulic result. Lessons on the failed cases of S-y are suggested to avoid disrupting factors in gravity surveys. A semiautomatic leveling procedure is proposed to maximize the chance for S-s determination. Our simulations show that gravity changes increase with S-y and decrease with initial hydraulic head and horizontal gravimeter-pumping well distance, helping to predict if the gravity-based method works at a site and to best plan a gravity survey. Drilling data show that the S-y values from the hydraulic and geodetic methods represent aquifer storage coefficients over different aquifer layers. Plain Language Summary A pumping test extracts groundwater from a well to estimate an aquifers storage coefficients, based on measurements of the groundwater lowering rate during pumping. But pumping can also lower gravity values and deform the surface. After a heavy rain stops, the aquifer level can rapidly decline to lower gravity values. We measure such gravity changes and surface deformations to determine hydrogeological parameters critical to understanding aquifer storage capacity and managing groundwater resource.
Absolute gravity change in Taiwan: Present result of geodynamic process investigation
(2017-12-01)
Kao, Ricky
;
Hwang, Cheinway
;
Kim, Jeong Woo
;
Ching, Kuo-En
;
Masson, Frederic
Gravity values at 24 sites over 2004-2016 measured with absolute gravimeters are used to study geodynamic processes in Taiwan. We model rain-induced gravity effects and other temporal effects of non-geodynamic origins to obtain residual gravity, which cannot be fully explained by GPS-derived vertical displacements. We explain the gravity changes associated with deposited debris, earthquake, volcanism and Moho deepening. Gravity changes of 53.37 and 23.38 mu Gal near Sinwulyu and Laonong Rivers are caused by typhoon Morakot, leading to estimated volumes of 6.0 x 10(5) and 3.6 x 10(5)m(3) in deposited debris. The observed co-seismic gravity change near the epicenter of the M 6.9 Pingtung earthquake (26 December 2006) is 3.12 +/- 0.99 mu Gal, consistent with a dislocation-based gravity change at the mu Gal level, thereby supplying a gravity constraint on the modeled fault parameters. The AG record at the Tatun Volcano Group is the longest, but large temporal gravity effects here has led to a current gravity signal-to-noise ratio of less than one, which cannot convince a sinking magma chamber, but supply an error bound for gravity detections of long-term or transient magma movements. The gravity values at Ludao and Lanyu decline steadily at the rates of -2.20 and -0.50 mu Gal yr(-1), consistent with the expected magma states of the two extinct volcanoes. The gravity rates at an uplifting site in central Taiwan and three subsiding sites in eastern Taiwan are negative, and are potentially caused by Moho deepening at a rate of -3.34 cm yr(-1) and a combined Moho deepening and plate subduction at the rates of -0.18, -2.03, and -1.34 cm yr(-1).
SOLID EARTH DEFORMATION MONITORING USING SATELLITE ALTIMETRY IN SOUTHWESTERN COASTS IN TAIWAN
(2010)
Cheng, Kai-chien
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Hwang, Cheinway
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Chung, Yu-Hsiang
;
Huang, Ching-Yao
;
Wei, Shiang-Hung
A novel method of using waveform retracked altimeter observations on land for the monitoring of the solid Earth deformation in Taiwan is described in the article. The southwestern coasts of Taiwan have been experiencing a significant land subsidence in the rate of similar to 8 cm/year, due to, in part, the excess extraction of the underground water. The first-order leveling networks and the GPS control points available in the area were used to verified the results of the waveform retracked altimeter observation. In this study, we proposed to use TOPEX/Poseidon (T/P) altimeter land observations from 1992-2002 on one T/P groundtrack that passes the southwestern coasts of Taiwan where is part of the Chia-Nan Plain with the main vegetation being rice paddy. This special vegetation and flat terrain provide an opportunity for successful waveform retracking. The comparison between T/P retracked result and that from the leveling network and GPS control points are presented.
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
;
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.
New free-air and Bouguer gravity fields of Taiwan from multiple platforms and sensors
(2014-01-25)
Hwang, Cheinway
;
Hsu, Hung-Jui
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Chang, Emmy T. Y.
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Featherstone, W. E.
;
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.
Numerical modeling of displacements due to ocean tide loading: case study at GPS stations in Taiwan and western Pacific
(2013)
Hwang, Cheinway
;
Huang, Jiu-Fu
;
土木工程學系
;
Department of Civil Engineering
We model the horizontal and radial displacements due to ocean tidal loading (OTL) in a computer program DISOTL (DISplacements due to Ocean Tide Loading). Numerical modeling of OTL considers inner and outer zone contributions. A local tide model and shoreline defined by a GMT and Taiwan digital elevation model is used for the inner zone. OTL-induced displacements from DISOTL, GOTIC2, and BS computer programs differ at the millimeter level in amplitude, but the phase difference can be over 10 degrees. Such displacements at 13 IGS stations in the western Pacific can be up to 8.5 cm in amplitude (KWJ1, Marshall Islands). At stations around Taiwan, the radial displacements can be up to 5.5 cm (MZUM, Matzu Islands). This implies that such large OTL effects (over 1 cm) will have a profound influence on precise global positioning system (GPS) positioning. To establish a new GPS reference frame in Taiwan, the use of a precise computer program is critical. A case study of coastal and offshore-island GPS continuous stations suggests that DISOTL can model OTL corrections to a 1 mm accuracy and reduce coordinate variations by up to 35%.
Vertical Displacement due to Ocean Tidal Loading Around Taiwan Based on GPS Observations
(2011-08-01)
Yeh, Ta-Kang
;
Hwang, Cheinway
;
Huang, Jiu-Fu
;
Chao, Benjamin Fong
;
Chang, Ming-Han
Ocean tidal loading (OTL) is an important factor in GPS positioning, especially along the vertical direction. OTL influences the precision of GPS positioning and produces height variations of up to 12 cm. In this study, daily UPS data obtained from 27 GPS tracking stations around Taiwan were collected, and four OTL models were applied to relative static GPS positions derived from these data. The UPS data were obtained from 8 - 14 August 2006 (summer) and 1 - 30 January 2007 (winter). The software Bernese 5.0 was utilized for data processing. During the summer, the average GPS error along the vertical axis was 2.72 cm. After correcting the measured heights with the Schwiderski, NAO.99b, CSR4.0 and TPXO.6.2 models, the average errors became 2.30, 2.30, 2.30 and 2.31 cm, respectively. In winter, the average UPS error in the vertical direction was 1.32 cm. After correcting the heights using the above models, the average errors became 0.86, 0.85, 0.89 and 0.84 cm, respectively. The precision of the vertical direction was improved by approximately 15 - 36% on average. In eastern Taiwan, all UPS stations in the vertical direction have amplitudes smaller than 1 cm, because the ocean is more than 2000 m deep and with less shallow and narrow bights and estuaries. On the other hand, the amplitudes of GPS stations in northwestern Taiwan are larger than 1 cm because the ocean is only approximately 200 m deep. The amplitudes in the vertical direction of UPS stations are approximately 1.2 - 2.7 cm on the offshore islands of Taiwan, larger than average in Taiwan. In the future, local corrections from an OTL model are necessary for high accurate GPS positioning.
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
;
Shih, Hsuan-Chang
;
Hsiao, Yu-Shen
;
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).
SGOTL: A Computer Program for Modeling High-Resolution, Height-Dependent Gravity Effect of Ocean Tide Loading
(2012-04-01)
Hwang, Cheinway
;
Huang, Jiu-Fu
;
土木工程學系
;
Department of Civil Engineering
SGOTL, a computer package coded in FORTRAN, has been developed to model the gravity effect due to ocean tide loading (OTL), especially for a coastal station with large ocean tides. SGOTL uses a regional and a global tide model to account separately for near (inner) and far (outer) zone contributions, and optimizes an inner-zone region and grid interval for numerical convolution. Height dependent Green's functions for Newtonian and elastic effects are developed. The coastline is defined by the full-resolution GMT shoreline, and optionally a digital elevation model (DEM). A case study using gravity observations at the Hsinchu superconducting gravity station and some offshore islands around the Taiwan Strait suggests that SGOTL outperforms some selected global OTL programs and achieves an accuracy of 0.1 mu Gal for 8 leading tidal constituents.

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