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Oceanography / Meteorology
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Automatic Detection of Oceanic Wave Length and Direction from SPOT Image
C.S.Wu, C.F. Chen, K.S. Chen and A.J. Chen
Center for Space and Remote Sensing Research
National Central University, Chung-Li, TAIWAN, R.O.C.
Abstract
An automatic method is developed to extract oceanic wave length and direction of major wave pattern near the coastal water from SPOT image. The process consists of three step: preprocessing, two-dimensional Fourier transformation, and wave detection. At first step, the wave appearance in the image is separated from the background in the image. Since the typical surface waves in boundaries are always found in the image due to the different water colors. The separation of wave patterns from the background becomes very important in the and unrelated linear features in the image, and then the probabilistic relaxation is image. At second step, fourier transform is utilized to transform the binary image from the spatial domain into the frequency domain for spectrum analysis. At the final step, the wave spectrum image in the frequency domain is analyzed by using filtering and clustering approach to detect the location of the governing power in the image. Finally, the length and direction of dominant ware is derived from frequency analysis. The method is tested by using 1993/12/8 SPOT PLA image along the coast of Tai-Chung Harbor, Western Taiwan. The wave lengths and directions vary from 70m to 66m and from 86o to 103o, respectively, result demonstrates that the trend of detected wave lengths and directions are consistent with the variation of ocean current and shoreline topography.
1. Introduction
It is apparent that the oceanic wave length and direction are two important ocean parameters in the oceanography. Conventionally, these data are collected by point-sampling from research vessel or moored buoys. These are time-consuming, costly and sometimes are dangerous. Satellite remote sensing presents a two-dimensional synoptic view and has the ability to provide a large-scale and long-period spatial-sampling data. However, the data volume being collected must be many order of magnitude greater than those being collected by traditional methods. Thus, an automatic detection system for remote sensing image will be of great help to the oceanographer.
In this paper, we propose a method which can automatically detect oceanic wave length and direction form SPOT image (The system diagram is shown in Fig. 1). The proposed method is divided into three stages : preprocessing, fourier transformation and wave detection. Because the wave phenomena captured by visible wavelength sensors, such as SPOT, is usually disturbed by the change of water color, or is obscured by the solar reflectance of ocean surface; the typical surface waves in the remotely sensed image normally appear to be dim and blurry and, moreover, the linear boundaries are always found in the image due to the different water colors. The separation of wave patterns from the background becomes very important in the first stop. This study uses the mathematical morphology to reduce the noise and unrelated linear features in the image, and then the probabilistic relaxation is employed to classify the wave patterns and transform the image into a binary image. At second step, the fourier transform is utilized to transform the binary step, the wave spectrum image in the frequency domains analyzed by using filtering and clustering approach to detect the location of the governing power in the image. As a result, the wave length and direction of dominant wave is derived wave is derived from frequency analysis.
Figure 1 The flow chart of the proposed system
In the next section, we present the details of our algorithm. Section 3 and 4 give experimental results and conclusions, respectively.
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