Browsing by Author "Lin, Wei-Tsun"

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Bathymetric LiDAR Green Channel Derived Reflectance: An Experiment from the Dongsha 2010 Mission
(2016-08)
Lin, Wei-Tsun
;
Shih, Peter Tian-Yuan
;
Chen, Jie-Chung
;
Liao, Chun-Jie
;
土木工程學系
Bathymetric LiDAR utilizes a green laser capable of penetrating water and surveying seafloor topography. The intensity of the echo identified as the seafloor carries information about the substrate type. However, besides the reflectance characteristics of the substrate, there are also other influencing factors, including those from the LiDAR system and the environment. The data collected in the 2010 Dongsha atoll bathymetric LiDAR mission is processed and analyzed in this study. The corrections for environmental factors, mainly contributed from the water column that is presented as the inherent optical parameters of water are retrieved from the recorded green laser waveform. Those from the system, such as deviations between the signal receptors or between laser beam angles to each interface, are eliminated with data from IMU (Inertial Measurement Unit), scanner controlling mechanisms, etc. The resulting reflectance from each flight line is compared in the overlap area. The reflectance of the west side strip is subtracted from that of the east side strip. The reflectance is scaled between zero and one. While the mean of the differences is -0.0037, the standard deviation is 0.0436 for the flight line with a flight height of 400 m. The mean and standard deviation are -0.0058 and 0.0272, respectively, with flight height of 300 m. When interpolating the reflectance from the 300 m flight altitude dataset into a surface after subtracting the point measurement from the 400 m flight altitude, the mean and standard deviations are -0.0215 and 0.0382, respectively. This indicates that the consistency among flight lines of the same flight altitude is higher than those from different flight altitudes. A WorldView-2 (WV-2) image is compared with the LiDAR reflectance, and after atmospheric correction, the green band reflectance from WV-2 showed high similarities between the two image types. However, in the deep water region the one derived from LiDAR has much more information content.
Bathymetric LiDAR Green Channel Derived Reflectance: An Experiment from the Dongsha 2010 Mission
(2016-08-01)
Lin, Wei-Tsun
;
Shih, Peter Tian-Yuan
;
Chen, Jie-Chung
;
Liao, Chun-Jie
;
土木工程學系
Bathymetric LiDAR utilizes a green laser capable of penetrating water and surveying seafloor topography. The intensity of the echo identified as the seafloor carries information about the substrate type. However, besides the reflectance characteristics of the substrate, there are also other influencing factors, including those from the LiDAR system and the environment. The data collected in the 2010 Dongsha atoll bathymetric LiDAR mission is processed and analyzed in this study. The corrections for environmental factors, mainly contributed from the water column that is presented as the inherent optical parameters of water are retrieved from the recorded green laser waveform. Those from the system, such as deviations between the signal receptors or between laser beam angles to each interface, are eliminated with data from IMU (Inertial Measurement Unit), scanner controlling mechanisms, etc. The resulting reflectance from each flight line is compared in the overlap area. The reflectance of the west side strip is subtracted from that of the east side strip. The reflectance is scaled between zero and one. While the mean of the differences is -0.0037, the standard deviation is 0.0436 for the flight line with a flight height of 400 m. The mean and standard deviation are -0.0058 and 0.0272, respectively, with flight height of 300 m. When interpolating the reflectance from the 300 m flight altitude dataset into a surface after subtracting the point measurement from the 400 m flight altitude, the mean and standard deviations are -0.0215 and 0.0382, respectively. This indicates that the consistency among flight lines of the same flight altitude is higher than those from different flight altitudes. A WorldView-2 (WV-2) image is compared with the LiDAR reflectance, and after atmospheric correction, the green band reflectance from WV-2 showed high similarities between the two image types. However, in the deep water region the one derived from LiDAR has much more information content.
Site Selection of Reference Tidal Station for National Height Datum in Taiwan
(2017-02)
Shih, Peter Tian-Yuan
;
Lin, Wei-Tsun
;
Shyue, Shiahn-Wern
;
Chen, Jie-Chung
;
Liao, Chun-Jie
National vertical data are mostly referenced to a tidal level of one or more tidal stations. The Keelung tidal station has been serving as the reference tidal station of the Taiwan National Height Datum for more than one century; all orthometric height in Taiwan is zeroed to the mean sea level (MSL) of an astronomical period of this station. A monument of the Taiwan height origin was established nearby, but due to construction, the height origin was relocated to the newly built National Museum of Marine Science and Technology in Badouzi. This article documents the search for an improved definition of the tidal reference for height origin in Taiwan. The selection criteria identified include geological stability; sufficient depth of water; limited influence from nontidal effects, such as passing vessels; and proximity to the monument of height origin. As part of the preliminary assessment, a tidal station was established at the nearby Cheng-Bin fishery port. The authors performed tidal comparisons of observations collected from the Keelung, Cheng-Bin, and nearby Longdong (to the east) and Taipei Harbor (to the west) stations. In addition to the harmonic analysis, tidal levels of different definitions were computed to determine the potential influence resulting from the change of reference tidal station to the national height datum. From concurrent observations over 3 months, from September 5 to December 17, 2013, the amplitude and phase of significant tidal components were found to be similar between the Keelung and Cheng-Bin stations, whereas the Longdong and Taipei Harbor stations presented larger deviations. The differences between MSL calculations were significant. Although the deviation between the national vertical datum zero and the tidal level was noted in previous studies, the Cheng-Bin station showed relatively smaller changes. Although defining height system with geoid and sea surface topography preserves a more physical and geometric definition of height, a MSL-based definition with a group of coordinated tidal stations may serve as a more reliable scheme for the transitional period.
UNDERWATER LINEAR FEATURE EXTRACTION WITH MULTISPECTRAL BAND IMAGES: AN EVALUATION WITH LEVEL-SET METHOD IN DONGSHAATOLL AND ZENGMU SHOAL
(2016-12-01)
Shih, Peter Tian-Yuan
;
Lin, Jian-Wei
;
Lin, Wei-Tsun
;
Wang, Cheng-Gi
;
土木工程學系
Optical remote sensing satellite images are a useful and convenient source to provide underwater features, particularly for shallow water areas because light, dependent on wavelength, has the capability to penetrate water. In this study, the information richness of underwater features is investigated for each spectral band of the optical images, and also several derived bands. This assessment is performed with the level-set method for segmentation. Two cases are analyzed in this study. The first study site is the Dongsha atoll, which is composed of Dongsha island, lagoon, and surrounding reefs. The water depth ranges from zero to less than 3 m at the outer ring and down to a depth of 20 m in the lagoon. The images were acquired with WorldView-2 in October 2013 and covered the entire atoll. The second study site is Zengmu shoal, an underwater feature. The image used is a scene acquired with Landsat 8. These images demonstrate high water clarity in both sites. For the Dongsha atoll, both the reflectance of each spectral band, the NDWI, and bands processed with Principle Component Transformation (PCT) are analyzed. The assessment is made based on the number of segments identified. The more segments identified, subsequently the more information, we assume, is provided. In order to remove those caused by noise, only the segments larger than 100 m(2) were counted. Based on this, PCT band 1 performs the best, and followed by green, yellow, coastal, blue, red, and fewer features from red-edge NIR and NIR2 bands when the objects in the scene are completely submerged underwater. For the Zengmu shoal, the boundary of the object identified is used for the assessment. The one closest to the manually. digitized imaged boundary would be recognized as having the best performance. Among the spectral bands, coastal/aerosol (CA) and blue perform the best. The four bands, coastal, blue, green, and red, are projected with PCT. The boundary resulting from the first principle component resembles most the one identified by a human operator on a QuickBird image.
UNDERWATER LINEAR FEATURE EXTRACTION WITH MULTISPECTRAL BAND IMAGES: AN EVALUATION WITH LEVEL-SET METHOD IN DONGSHAATOLL AND ZENGMU SHOAL
(2016-12)
Shih, Peter Tian-Yuan
;
Lin, Jian-Wei
;
Lin, Wei-Tsun
;
Wang, Cheng-Gi
;
土木工程學系
Optical remote sensing satellite images are a useful and convenient source to provide underwater features, particularly for shallow water areas because light, dependent on wavelength, has the capability to penetrate water. In this study, the information richness of underwater features is investigated for each spectral band of the optical images, and also several derived bands. This assessment is performed with the level-set method for segmentation. Two cases are analyzed in this study. The first study site is the Dongsha atoll, which is composed of Dongsha island, lagoon, and surrounding reefs. The water depth ranges from zero to less than 3 m at the outer ring and down to a depth of 20 m in the lagoon. The images were acquired with WorldView-2 in October 2013 and covered the entire atoll. The second study site is Zengmu shoal, an underwater feature. The image used is a scene acquired with Landsat 8. These images demonstrate high water clarity in both sites. For the Dongsha atoll, both the reflectance of each spectral band, the NDWI, and bands processed with Principle Component Transformation (PCT) are analyzed. The assessment is made based on the number of segments identified. The more segments identified, subsequently the more information, we assume, is provided. In order to remove those caused by noise, only the segments larger than 100 m(2) were counted. Based on this, PCT band 1 performs the best, and followed by green, yellow, coastal, blue, red, and fewer features from red-edge NIR and NIR2 bands when the objects in the scene are completely submerged underwater. For the Zengmu shoal, the boundary of the object identified is used for the assessment. The one closest to the manually. digitized imaged boundary would be recognized as having the best performance. Among the spectral bands, coastal/aerosol (CA) and blue perform the best. The four bands, coastal, blue, green, and red, are projected with PCT. The boundary resulting from the first principle component resembles most the one identified by a human operator on a QuickBird image.