Preliminary investigation of determining wave rays and bathymetry using satellite images

• Main Author: 柯紳彥
• Other Authors: 張憲國
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/07959671369461720971

碩士 === 國立交通大學 === 土木工程學系 === 100 === This paper presents a new method of determining wave rays and bathymetry from satellite images instead of the previous bathymetry survey using a vehicle with single/multi-beam echo sounder. Some techniques of image processing are used to obtain wave rays and water depth. The techniques include: (1) histogram equalization and spatial filtering for increasing image contrast; (2) image erosion for filtering out the noise; (3) Otsu algorithm used for the threshold of extracting crest lines; (4) some algorithms of determining wave rays normal to crest lines; (5) corrected wavelength on one wave ray between two adjacent fronts; (6) neglecting wave acoustics water depth determined by the dispersion relation of the Airy wave theory with known the wave period and wave length. Finally, the whole bathymetry can be obtained at the intersection between all determined crest lines and wave rays. The proposed method is assessed valid for estimating wave rays and water depth by comparing the results obtained by a numerical model and by bathymetry survey with those obtained by the proposed method at the Pingtung coast. The root mean square error and the mean absolute error between the estimated wave rays and those calculated by the numerical model are about 4.5 degrees and 3 degrees. The estimated water depths by the proposed methods are lower than those by the bathymetry survey for water depth from -5m to -7.5m. However, the estimated water depths by the proposed methods are slightly higher than those by the bathymetry survey for deeper depth from -7.5m to -10m. Both results are approximate when water depth exceeds -10m. The proposed method cannot exactly and accurately identify the existence of offshore bars. The root mean square error between the estimated water depths and measured ones along a non-dune profile is 0.62m and is 0.9m.along a dune profile. The proposed method of determining wave rays is corrected to the case in the diffracted zone according to the characteristics of crest lines in the diffracted zone of the theory of water wave diffraction. The corrected method of wave rays is examined good in the diffracted zone of the Guishan Island.