Browsing by Author "Shih, Peter Tian-Yuan"

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A GEOMORPHOLOGICAL MODEL FOR LANDSLIDE DETECTION USING AIRBORNE LIDAR DATA
(2012-12-01)
Liu, Jin-King
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Hsiao, Kuo-Hsin
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Shih, Peter Tian-Yuan
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土木工程學系
;
Department of Civil Engineering
This study analyzes multi-temporal LiDAR data of high accuracy and high resolution by installing a geomorphometric model for extracting landslides. First, two sets of LiDAR data were acquired for before and after a heavy rainfall event. The landslides which took place from 2005 to 2009 were classified automatically by satellite images, and subsequently the landslides were interpreted and edited manually. Geomorphometric parameters including slope, curvature, OHM, OHM roughness, and topographic wetness index were then extracted using stencils of landslide polygons overlaid on respective thematic maps derived from LiDAR, DEM and DSM. The ranges of every parameter were derived from the statistics of the landslide area. Some selected non-morphometric parameters were also included in a later stage to account for all possible features of landslides, such as vegetation index and geological strength. The ranges of the parameters of landslides were optimized for the model by the statistics of the landslide area. The overall accuracy predicted by the model was 64.9%. When the buffer zones of old landslides and riverside areas were included, the overall accuracy was 64.4%, showing no improvement. When landslides smaller than 50 m(2) were filtered, the 2009, respectively. The results show that the geomorphological model proposed in this research is effective for landslide extraction.
A Study on Factors Affecting Airborne LiDAR Penetration
(2015-04-01)
Hsu, Wei-Chen
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Shih, Peter Tian-Yuan
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Chang, Hung-Cheng
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Liu, Jin-King
;
土木工程學系
This study uses data from different periods, areas and parameters of airborne LiDAR (light detection and ranging) surveys to understand the factors that influence airborne LiDAR penetration rate. A discussion is presented on the relationships between these factors and LiDAR penetration rate. The results show that the flight height above ground level (AGL) does not have any relationship with the penetration rate. There are some factors that should have larger influence. For example, the laser is affected by a wet ground surface by reducing the number of return echoes. The field of view (FOV) has a slightly negative correlation with the penetration rate, which indicates that the laser incidence angle close to zero should achieve the best penetration. The vegetation cover rate also shows a negative correlation with the penetration rate, thus bare ground and reduced vegetation in the aftermath of a typhoon also cause high penetration rate. More return echoes could be extracted from the full-waveform system, thereby effectively improving the penetration rate. This study shows that full-waveform LiDAR is an effective method for increasing the number of surface reflected echoes. This study suggests avoiding LiDAR survey employment directly following precipitation to prevent laser echo reduction.
AN INTRODUCTION OF SURVEYING AND GEOMATICS EDUCATION WITH E-PLATFORM IN NCTU, TAIWAN
(2011-01-01)
Teo, Tee-Ann
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Shih, Peter Tian-Yuan
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土木工程學系
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Department of Civil Engineering
This article presents the current status of Surveying and Geomatics education provided by Department of Civil Engineering at National Chiao Tung University, Taiwan. The Surveying and Geomatics Education at NCTU is introduced first. Then the current status of using E-learning platform for Surveying and Geomatics courses is described. This platform, also known as E-Campus, is designed and implemented by NCTU Digital Content Production Center. This paper also shows some statistical numbers of Surveying and Geomatics courses using E-Campus. The practical results indicated that the average login for undergraduate student is from 38 to 60 times per student for each course while the average login for graduate student is from 69 to 105.
Bathymetric LiDAR Green Channel Derived Reflectance: An Experiment from the Dongsha 2010 Mission
(2016-08)
Lin, Wei-Tsun
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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.
Bathymetric Lidar Survey Of Penghu Islands and Dongsha Atoll Using an Ellipsoidal Height System for Bathymetric Mapping In Shallow Waters and Difficult-to-Navigate Environments
(2011-11-01)
Shih, Peter Tian-Yuan
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Arumugam, Dushan
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Shyue, Shiahn-Wern
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防災與水環境研究中心
;
Disaster Prevention and Water Environment Research Center
Cloud component removal for shallow water depth retrieval with multi-spectral satellite imagery
(2019-08-01)
Tsou, Po-Yao
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Shih, Peter Tian-Yuan
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Teo, Tee-Ann
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土木工程學系
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Department of Civil Engineering
Water depth and the topography under water provide important information for near shore human activities. With the intensification of international territory concerns, bathymetric mapping is also gaining attention. Optical satellite imagery is an efficient tool for estimating shallow water depth as compared to the traditional field surveying because of its wide coverage area. On the other hand, the existence of cloud and haze contaminates the spectral signatures, which introduces errors to the depth data retrieved. In this research, the contaminated pixels are treated as a mixture of water and cloud. Linear Spectral Unmixing (LSU) procedure is applied for estimating the cloud abundance in mixed pixels. The cloud component is then removed with a linear function and the "purified" water component used for depth retrieval. In this research, water depth is estimated with two methods, namely, artificial neural network (ANN) and physical model. The former demands in-situ bathymetric samples for training, the latter requires site information of inherent optical properties (IOPs) and apparent optical properties (AOPs). The experiments reveal that retrieving depth with ANN generates better results than the physical model, but with a few extremely large errors. As for mixed pixels, the error of depth estimation becomes higher as cloud abundance increases. The precision of depth retrieval is higher for mixed pixels at the reef flat level (within 10 m in depth) than those in the lagoon (about 20 m in depth), and the precision generally agrees with those retrieved from water pixels without cloud or haze.
Evaluating Information Loss of SRTM DEM Data with Different Grid
(2014-11-01)
Shih, Peter Tian-Yuan
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土木工程學系
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Department of Civil Engineering
The digital elevation products produced by the Shuttle Radar Topography Mission (SRTM) are disseminated with different resolutions. This study investigates the differences in information content between different resolutions. Three 1 degrees cells are selected to represent flat area, moderate terrain, and mountain ranges. All of these cells are located in the continental territory of the United States. The theory of information loss caused by the sampling density is shown with the increasing deviation from the reference in the experiment. The error resulting from bilinear interpolation is also assessed. The experimental result also indicates that if the 1-arcsecond SRTM, digital elevation model (DEM) is released for global coverage, the height accuracy would be improved similar to 10% over the 3-arcsecond product. (C) 2014 American Society of Civil Engineers.
Geomorphological Change Detection Using an Integrated Method: A Case Study on the Taan River Watershed, Taiwan
(2016-08-01)
Wu, Jyun-Yi
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Teo, Tee-Ann
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Shih, Peter Tian-Yuan
;
土木工程學系
;
Department of Civil Engineering
The Chi-Chi earthquake of 1999 in Central Taiwan caused significant vertical uplift and ignited a dynamic fluvial erosion system in the Taan River. In order to understand the geomorphologic changes in the Taan River after the Chi-Chi earthquake, this study uses multi-temporal aerial photos to produce orthoimages and surface models to derive regional three dimensional movements from 2001 - 2009. The Particle Image Velocimetry (PIV) technique is applied to identify the horizontal movements from both orthoimages and digital surface models (DSMs). While PIV can provide estimates for the channel change direction and magnitude; vertical analysis such as river incision rate derivation could be performed using the height difference between multi-temporal surface models. The results from these two approaches are then integrated for river geomorphological interpretation. It is observed that the proposed scheme is capable of identifying three river evolution stages from the extracted movements in the Taan case. These stages are: years 2001 to 2003, 2004 to 2006, and 2007 to 2009, respectively. The total incision between 2001 and 2009 is about 10 m. This study demonstrates that both PIV and DSM subtraction are effective in river geomorphological change identification. The integration of these two approaches could provide more information when observing the evolution of river morphology.
Geomorphological Change Detection Using an Integrated Method: A Case Study on the Taan River Watershed, Taiwan
(2016-08)
Wu, Jyun-Yi
;
Teo, Tee-Ann
;
Shih, Peter Tian-Yuan
;
土木工程學系
;
Department of Civil Engineering
The Chi-Chi earthquake of 1999 in Central Taiwan caused significant vertical uplift and ignited a dynamic fluvial erosion system in the Taan River. In order to understand the geomorphologic changes in the Taan River after the Chi-Chi earthquake, this study uses multi-temporal aerial photos to produce orthoimages and surface models to derive regional three dimensional movements from 2001 - 2009. The Particle Image Velocimetry (PIV) technique is applied to identify the horizontal movements from both orthoimages and digital surface models (DSMs). While PIV can provide estimates for the channel change direction and magnitude; vertical analysis such as river incision rate derivation could be performed using the height difference between multi-temporal surface models. The results from these two approaches are then integrated for river geomorphological interpretation. It is observed that the proposed scheme is capable of identifying three river evolution stages from the extracted movements in the Taan case. These stages are: years 2001 to 2003, 2004 to 2006, and 2007 to 2009, respectively. The total incision between 2001 and 2009 is about 10 m. This study demonstrates that both PIV and DSM subtraction are effective in river geomorphological change identification. The integration of these two approaches could provide more information when observing the evolution of river morphology.
Historic Shipwreck Study in Dongsha Atoll with Bathymetric LiDAR
(2014-04-01)
Shih, Peter Tian-Yuan
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Chen, Ya-Hsing
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Chen, Jie-Chung
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土木工程學系
;
防災與水環境研究中心
Dongsha Atoll is a coral reef located in the South China Sea. The surrounding area is characterized by dangerous shoals. Historic shipwrecks mark past human activities. Due to the shallow water and risky navigational conditions in the area, a sonar survey with platforms on the water surface was not feasible. Airborne bathymetric LiDAR, which utilizes green laser for measurement, however, is a proven convenient method for studying shipwrecks around the atoll, particularly in shallow-water areas. At a point density of about 3.5m by 3.5m, four shipwrecks were identified. The bathymetric measurements allow not only the length of the shipwreck to be estimated, but also its height above the sea floor. The full waveform record of the laser reflection also provided information to separate the wreckage from its surroundings. This provides an excellent working environment for marine archaeological analysis, as not only the location, but also the depth and geomorphological information can be assessed in an integrated setting. Copyright (c) 2013 John Wiley & Sons, Ltd.
Introduction to the Special Issue on "Oceanic and Coastal Remote Sensing" of Journal of Marine Science and Technology
(2016-12)
Shih, Peter Tian-Yuan
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Ho, Chung-Ru
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Tseng, Yi-Hsing
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Lin, Tang-Huang
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交大名義發表
Land Cover Classification Accuracy Assessment Using Full-Waveform LiDAR Data
(2015-04-01)
Chang, Kuan-Tsung
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Yu, Feng-Chi
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Chang, Yi
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Hwang, Jin-Tsong
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Liu, Jin-King
The geomorphology of Taiwan is characterized by marked changes in terrain, geological fractures, and frequent natural disasters. Because of sustained economic growth, urbanization and land development, the land cover in Taiwan has undergone frequent use changes. Among the various technologies for monitoring changes in land cover, remote sensing technologies, such as LiDAR, are efficient tools for collecting a broad range of spectral and spatial data. Two types of airborne LiDAR systems exist; full-waveform (FW) LiDAR and traditional discrete-echo LiDAR. Because reflected waveforms are affected by the land object material type and properties, the waveform features can be applied to analyze the characteristics specifically associated with land-cover classification (LCC). Five types of land cover that characterize the volcanic Guishan Island were investigated. The automatic LCC method was used to elucidate the spectral, geomorphometric and textural characteristics. Interpretation keys accompanied by additional information were extracted from the FW LiDAR data for subsequent statistical and separation analyses. The results show that the Gabor texture and geomorphometric features, such as the normalized digital surface model (nDSM) and slopes can enhance the overall LCC accuracy to higher than 90%. Moreover, both the producer and user accuracy can be higher than 92% for forest and built-up types using amplitude and pulse width. Although the waveform characteristics did not perform as well as anticipated due to the waveform data sampling rate, the data provides suitable training samples for testing the waveform feature effects.
Landslide monitoring with interferometric SAR in Liugui, a vegetated area
(2019-08-01)
Shih, Peter Tian-Yuan
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Wang, Huijung
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Li, Kuo-Wei
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Liao, Jyh-Jong
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Pan, Yii-Wen
Interferometric SAR analysis with a time series of images obtained from satellite has been proven to be a useful monitoring scheme for geomorphological changes, particularly related to the vertical dimension. The success of this technique relies on the coherence between the images. This imposes severe challenges for applications in vegetated areas. In this study, the landslide process of a site located in Liugui, southern Taiwan, is investigated by a time series of SAR images collected from ALOS PALSAR and Sentinel 1A. This landslide monitoring site is heavily vegetated. From the Persistent Scattering (PS) analysis, ALOS PALSAR presented more extracted points. The wavelength of the band may be a decisive factor. The time series of ALOS images spans across the deadliest typhoon impacted Taiwan in recorded history, Typhoon Morakot on 8 August 2009. It is clear that there are more PS points extracted from the time series after the event as compared to before Typhoon Morakot. This is largely explained by the increase in the amount of bare earth after the disaster. From the interferometric SAR analysis, the development of headward erosion could be observed. Although the number of PS points is limited, the understanding of erosion and landslide progress in the area could still be improved while integrating with the digital elevation model collected with airborne lidar, historic aerial photos, and images collected with UAS (Unmanned Aviation Systems). Together with the in situ measurements, the colluvium depth ranges from 2 - 40 m in the area. The surface instability is expected to persist.
National Datum Uncertainty due to Reference Frame Transformation: Case Study for the Geodetic Datum of Taiwan
(2014-08-01)
Chiu, Yuan-Hung
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Shih, Peter Tian-Yuan
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土木工程學系
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Department of Civil Engineering
With the advent of global navigation satellite systems (GNSSs), many national geodetic datums have been established with modern technology and referenced to a specific epoch of the international terrestrial reference frame (ITRF) definition. Following the static datum concept, the datum remains unchanged after its publication, which consequently creates problems in datum maintenance. In the recent update of TWD97, the national datum presently used in Taiwan, the resurvey coordinates are transformed back to the original frame but expressed in a recent epoch. In this study, the uncertainties resulting from the transformation between ITRF definitions in the region of Taiwan were investigated. Based on error propagation, a standard error of +/- 7 cm was estimated due to the transformation of the TWD97 coordinates between ITRF2005(2010.0) and ITRF94(2010.0). The uncertainties propagated from transformation parameters are more significant than those from station coordinates and velocities, and the uncertainty expands with the time interval. The SD is +/- 1.1 cm for the transformation between ITRF2005 and ITRF2000. Considering that the uncertainty between later definitions is much smaller than transforming to ITRF94, it may be preferable to replace the ITRF94 with a modern ITRF definition, at least ITRF2000, for the next TWD97 datum maintenance. (C) 2014 American Society of Civil Engineers.
Preface to the Special Issue on "Innovative Applications of Radar and LiDAR Remote Sensing"
(2016-08)
Shih, Peter Tian-Yuan
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Tseng, Yi-Hsing
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Tsai, Fuan
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Chang, Chung-Pai
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土木工程學系
Site Selection of Reference Tidal Station for National Height Datum in Taiwan
(2017-02)
Shih, Peter Tian-Yuan
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Lin, Wei-Tsun
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Shyue, Shiahn-Wern
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Chen, Jie-Chung
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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.
The building shadow problem of airborne lidar
(2009-12-01)
Shih, Peter Tian-Yuan
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Huang, Ching -Mei
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交大名義發表
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National Chiao Tung University
Airborne lidar systems are equipped with a scanning mechanism that produces a swath of 3D coordinated points to form a cloud in each flight strip. The scanning is perpendicular to the direction of flight. The density of the point cloud is one of the major quality measures for the data collected with such systems. Point density, shade (occlusion of the laser beam) and penetration rate are major factors to be considered in the flight design. For practical operations, flight missions may be designed with overlaps ranging from 5 to 50%. Aside from the percentage overlap, the field of view is another important parameter for flight design. An evaluation scheme to assess the percentage overlap between flight strips and the selection of the field of view is proposed in this study. The influences of overlap and field of view are also demonstrated with real data gathered by an airborne lidar system.Resume Les systemes lidar aeroportes sont equipes d'un mecanisme de balayage qui engendre un couloir de nuages de points pour chaque ligne de vol. Le balayage est perpendiculaire la direction de vol. La densite du nuage de point est l'un des principaux criteres de qualite pour les donnees acquises par de tels systemes. La densite des points, l'ombre et le taux de penetration sont les principaux facteurs prendre en compte dans la planification d'un vol. Pour des operations classiques, les missions de vol peuvent etre concues avec des recouvrements compris entre 5 et 50%. De meme que le pourcentage de recouvrement, le champ de vue est un autre parametre important pour la conception du vol. Cette etude propose une methode d'evaluation pour guider le choix du pourcentage de recouvrement entre couloirs de vol et celui du champ de vue. Les influences du recouvrement et du champ de vue sont aussi demontrees avec des donnees reelles issues d'un systeme lidar aeroporte.Zusammenfassung Flugzeuggestutzte Lidarsysteme sind mit einem Scanmechanismus ausgestattet, der fur jeden Flugstreifen eine Punktwolke in Schwadbreite generiert. Die Scanrichtung ist senkrecht zur Flugrichtung. Die Dichte der Punktwolke ist eines der wesentlichen Qualitatsma ss e fur die erfassten Daten. Punktdichte, Abschattung und Durchdringungsrate sind wesentliche Faktoren, die bei der Flugplanung berucksichtigt werden mussen. Fur die Anwendungen in der Praxis werden Uberlappungen von 5 bis 50% bei der Flugplanung vorgesehen. Neben der prozentualen Uberlappung ist aber auch das Sichtfeld ein weiterer wichtiger Parameter fur die Flugplanung. In dieser Studie wird ein Schema zur Bewertung der prozentualen Uberlappung zwischen den Flugstreifen und der Auswahl des Sichtfeldes vorgestellt. Deren Einflusse werden auch an realen Daten eines flugzeuggestutzten Lidarsystems demonstriert.Resumen Los sistemas lidar aerotransportados van equipados con un mecanismo de escaneado que traza una franja de nubes de puntos en cada pasada de vuelo. El escaneado es perpendicular a la direccion de vuelo. La densidad de la nube de puntos es una de las principales medidas de calidad de los datos obtenidos con estos sistemas. La densidad de puntos, la sombra y la tasa de penetracion son los factores mas importantes a tener en cuenta en la planificacion del vuelo. Por razones operativas las misiones de vuelo se disenan con recubrimientos que oscilan entre un 5 y un 50%. Aparte del porcentaje de solapamiento, el angulo de campo es otro parametro importante a tener en cuenta. Este estudio aporta unos procedimientos de evaluacion del recubrimiento entre pasadas y de seleccion del angulo de campo. La influencia de estas dos variables se ha determinado con datos de campo obtenidos mediante un sistema lidar aerotransportado.
THE USE OF UAS FOR RAPID 3D MAPPING IN GEOMATICS EDUCATION
(2016)
Teo, Tee-Ann
;
Shih, Peter Tian-Yuan
;
Yu, Sz-Cheng
;
Tsai, Fuan
;
土木工程學系
With the development of technology, UAS is an advance technology to support rapid mapping for disaster response. The aim of this study is to develop educational modules for UAS data processing in rapid 3D mapping. The designed modules for this study are focused on UAV data processing from available freeware or trial software for education purpose. The key modules include orientation modelling, 3D point clouds generation, image georeferencing and visualization. The orientation modelling modules adopts VisualSFM to determine the projection matrix for each image station. Besides, the approximate ground control points are measured from OpenStreetMap for absolute orientation. The second module uses SURE and the orientation files from previous module for 3D point clouds generation. Then, the ground point selection and digital terrain model generation can be archived by LAStools. The third module stitches individual rectified images into a mosaic image using Microsoft ICE (Image Composite Editor). The last module visualizes and measures the generated dense point clouds in CloudCompare. These comprehensive UAS processing modules allow the students to gain the skills to process and deliver UAS photogrammetric products in rapid 3D mapping. Moreover, they can also apply the photogrammetric products for analysis in practice.
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
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Lin, Jian-Wei
;
Lin, Wei-Tsun
;
Wang, Cheng-Gi
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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.