應用衛星影像決定波向線及估算水深之初步探討

Abstract

不同於往昔利用船隻載具單束或多束測深儀來量測海底地形，本文提出以光學衛星影像來決定波向線及估算水深的新技術。本文主要運用直方圖均化及空間濾波提高影像對比度，以影像侵蝕濾除雜訊，再採用Otsu演算法求得波紋辨識之門檻值，擷取影像圖中的波紋線。垂直於波紋線的定義決定波向線，本文在不考慮因波浪交會能量集中產生波峰變化的情況，以平滑化修正波長，並運用波浪理論中的分散關係式估算海底水深。 本文利用屏東海岸實測地形評估本模式決定的波向角度及水深之精度，以本模式推算的波向角度與數模計算所得比較二者的誤差均方根約為4.5度，平均絕對值誤差約為3度，顯示本方法具有可靠的精度來決定波向角度。至於估算水深方面，本模式估算於-5m至-7.5m間的水深較實測水深低，但-7.5m至-10m的水深卻略高，而水深-10m以下的情況，二者估算值接近。本法尚無法完全確實辨認外灘有無沙堆存在。因此，當無外灘沙堆的剖面，估算水深與實測水深的誤差均方根約為0.62m，當有外灘沙堆的剖面，估算水深與實測水深的誤差均方根升高約至0.9m。 在明顯繞射區域本文所提出波向需要依繞射理論之波峰線特性修正擷取波紋線之方法。並以龜山島地形遮蔽所產生的波浪繞射現象驗證本文所提決定波浪繞射區的波向線方法是可行的。

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.