| Summary: | 碩士 === 國立交通大學 === 土木工程系所 === 101 === In the past, the way to get bathymetry data was using vessels measuring point by point although the vessel was equipped with a GPS positioning system. However, route planning on the actual measurements was still unable to grasp accurately; therefore, this paper is going to illustrate how Fast Fourier Transform on the satellite images can determine wave rays and bathymetry.
This paper choose Guishan Island and Pingtung Fangliao coast as research base. The Guishan Island coast water changed significantly, which makes it clearer to observe the wave characteristics. On the other hand, Fanliao Sea coastline is straight without twists. The wave characteristics are not obvious, but there are bathymetric data available for verification.
Some techniques of image processing are used to obtain wave rays and water depth. The techniques include: (1) histogram equalization for increasing image contrast; (2) Butter Worth high-pass filter for filtering out the noise; (3) Determining wave rays and wavelength by using spectrum. Finally, consider waves passed to the shore gently, using theory of wave refraction correction wavelength and estimate bathymetry.
The proposed method is assessed valid for estimating water depth by comparing the results obtained by bathymetry survey with those obtained by the proposed method at the Pingtung coast. The estimated water depths by the proposed methods are from -4m to -10m, which is higher than the data by the bathymetry survey. However, for deeper water depth exceeds -12m, the estimated water depths by the proposed methods are slightly lower than those by the bathymetry survey. Both results are approximate between the water depth from -10m to -12m. The root mean square error between the estimated water depths and measured ones along a profile is 0.62m. If we don’t consider the part of messy wavy lines, its root mean square error is reduced to 0.31m. The results indicate that the method of estimating the water depth has certain credibility. In the future, this method can be used to harbor construction planning.
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