Browsing by Author "Huang, Jiu-Fu"

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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
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Hsu, Hung-Jui
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Featherstone, W. E.
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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.
Numerical modeling of displacements due to ocean tide loading: case study at GPS stations in Taiwan and western Pacific
(2013)
Hwang, Cheinway
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Huang, Jiu-Fu
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土木工程學系
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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%.
Results from parallel observations of superconducting and absolute gravimeters and GPS at the Hsinchu station of Global Geodynamics Project, Taiwan
(2009-07-21)
Hwang, Cheinway
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Kao, Ricky
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Cheng, Ching-Chung
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Huang, Jiu-Fu
;
Lee, Chiung-Wu
The Hsinchu (HS) superconducting gravimeter (SG, serial T48) station is a newly established site in the Global Geodynamics Project (GGP). Simultaneous observations of T48, three FG5 absolute gravimeters, and GPS at four stations are studied. GPS shows few mm a(-1) of horizontal and vertical motions around HS. The calibration factor and drifting rate of T48 are -75.96 +/- 0.07 mu Gal V(-1) and 0.2 +/- 0.7 mu Gal a(-1) (1 mu Gal = 10(-8) m s(-2)). Both the SG and absolute gravity records contain trends of about 2-3 mu Gal a(-1). The ocean tide gravity effects (OTGEs) were estimated from NAO. 99b, FES2004, and CSR4.0, and their amplitudes agree with the SG observations at the submicroGal level, but their phases differ from the observations up to 10 degrees. The Newtonian effect of ocean tide contributes 20% to the total OTGE at HS, and it is larger at islands in the Taiwan Strait. The inelastic body tide model of Dehant et al. (1999) is more consistent with the SG observations than the elastic model. Modeled gravity-atmosphere admittances based on an exponential distribution of air mass explain well the observed admittances. The average gravity-atmosphere admittance during typhoons is 30% larger than that in a nontyphoon time. A list of coseismic gravity changes from T48 caused by earthquakes over 2006-2007 is given for potential studies of fault parameters. The modeled effects of atmospheric pressure, groundwater, soil moisture, and polar motion explain the FG5 observed gravity trend to 1.1 mu Gal a(-1). Seasonally, the groundwater-induced gravity change contributes the most to the SG residual gravity, but its phase leads the latter by 63 days.
SGOTL: A Computer Program for Modeling High-Resolution, Height-Dependent Gravity Effect of Ocean Tide Loading
(2012-04-01)
Hwang, Cheinway
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Huang, Jiu-Fu
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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.
Vertical Displacement due to Ocean Tidal Loading Around Taiwan Based on GPS Observations
(2011-08-01)
Yeh, Ta-Kang
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Hwang, Cheinway
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Huang, Jiu-Fu
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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.