應用X-band航海雷達於海岸灘線測量之可行性研究

海岸變遷現象目前已普遍發生在臺灣的海岸地區，要瞭解海岸變遷的原因，先要觀察與掌握海岸變化的狀況，以避免導致錯誤決策。由於海岸地形侵淤的變化常明顯表現在海岸灘線位置的前進與後退，因此灘線位置的量測便顯得重要。灘線會隨著波浪的進退與潮汐的漲落而會不斷地改變其位置，具有高度的天然變動性，且海岸變遷的範圍又會隨不同時間尺度而變化，因此本研究以岸基X-band航海雷達作為海岸變遷的監測工具，可於定點針對固定空間進行全天候觀測，若能研發適當的雷達回波影像分析技術，將可達到即時與長期掌握大範圍海岸變化狀況之效。本研究之目的是要利用X-band雷達回波影像來測量灘線位置，根據雷達回波的原理與機制，以雷達回波強度之時間平均與標準偏差統計值可以突顯出海面與陸地所呈現之不同回波訊號特性，因此本文提出一套利用X-band雷達進行灘線位置量測之處理程序與分析流程，可將雷達回波平均影像經由邊緣偵測與形態學方法之技術整合，藉此判別海面與濕沙交界之灘線位置；或以雷達回波標準偏差影像透過影像濾波、二值化處理、邊緣偵測、及形態學之處理，可辨識出水陸交界線之灘線位置。為證實雷達應用於海岸灘線測繪之可行性，本研究利用無人飛行載具拍攝之光學影像與之比較，可發現雷達影像所量測的灘線位置與光學影像中目測辨識的灘線位置相近。將雷達影像進行兩種不同的灘線識別演算，可發現雷達回波標準偏差影像量測灘線之方法較不適用於風平浪靜的海況條件下；相較之下，雷達回波平均影像量測灘線之方法則不受海況的影響，因此本研究後續是運用雷達回波平均影像進行灘線量測，探討台南安平商港南側海岸之不同時期的灘線變化。以2017年5月、2017年11月、及2018年3月所觀測的雷達影像為例，選取各觀測期間最接近平均潮位時的雷達回波影像序列，進行平均影像的灘線量測與比較，可發現安平商港南側海岸有季節性灘線變遷之現象，但在此期間此段海岸之灘線變化呈平衡穩定狀態。以X-band雷達進行海岸灘線監測之優勢是能進行即時且連續地觀測，即使在天候不佳的狀況下，也能提供遠端監控服務，因此能夠長期、持續地自動監測海岸灘線之時空變化，以協助權責單位掌握海岸變遷現況，使有效地進行海岸地區的防治及永續發展工作，並制定相關的管理辦法。

關鍵字

灘線量測；影像處理與辨識；邊緣偵測； X-band雷達

並列摘要

The purpose of this study is to estimate shoreline positions from X-band radar echo images. Based on the principle and mechanism of radar echo signals, the time-averaged and standard deviation statistics of radar echo intensities can highlight the different characteristics of the sea and land. Therefore, this study proposes a set of processing procedures for the measurement of shoreline positions using X-band marine radar. The proposed technique integrates an image filter, thresholding, edge detection, and a morphological method, which are used to determine the shoreline positions at the land-water interface. In order to assess the feasibility of the proposed method, the obtained shoreline positions are compared with those in an optical image from an unmanned aerial vehicle (UAV). It was found that the shoreline positions measured from radar images agree with those visually identified from UAV orthoimages. The shoreline positions derived from time-averaged radar images were compared with those derived from standard deviation radar images. The results show that shoreline measurements using time-averaged radar images are more suitable than those using standard deviation radar images under calm wave conditions, and that the former are unaffected by sea surface conditions. Therefore, time-averaged radar images were selected for shoreline measurements to discuss shoreline variations at the southern sandy coast of Anping commercial port in Tainan during various periods, and to demonstrate the practicability of the proposed method. The advantage of coastal shoreline monitoring with X-band radar is that can provide real-time and uninterrupted observation, even in bad weather conditions. Therefore, the temporal and spatial variations of a shoreline can be automatically and continuously monitored over the long term to help authorities understand coastal changes, facilitating coastal protection and sustainable development in coastal zones.