| 摘要: | 測深對於海岸的研究、資源的利用或航行探勘都是很重要的一項工作，而傳統上為了取得較高精度的水深，常見的方法是由空載光達或船載回波測深儀測量水深，但是其成本高昂，且受天候、航行安全性及測區易達性等影響及限制，水深資料往往不易取得。近年隨著衛星遙感技術的發展，美國NASA於2018年底發射ICESat-2 (The Ice, Cloud, and Land Elevation Satellite-2)雷射測高衛星，證實可於濁度較低的潛水區作為測深光達，提供穩定的測距資料。ICESat-2搭載532nm綠光波段的高精度雷射測高系統ATLAS (the Advanced Topographic Laser Altimeter System)，擁有每秒10 kHz脈衝頻率，以及可同時發射6條綠光雷射光至地表。儘管ICESat-2資料具備高解析力，然而ICESat-2僅提供沿軌道的高程剖面資料，在平行的地面軌道之間留有數據間隙。因此，本研究結合ICESat-2觀測資料及多光譜光學衛星影像，它能夠根據光譜衰減行為得出完整的水深圖。
這項研究的目的是結合ICESat-2資料和Sentinel-2光學衛星影像，反演南海六個島礁和潟湖的淺水深度（深度20 m以內）。研究首先對ICESat-2 ATL03光子點雲資料進行濾波，以找到沿著軌道的水底剖面。並且與現地空載LiDAR的測量結果相驗證，得到均方根誤差（RMSE）在0.20–0.47 m範圍內。接下來，使用三個半經驗函數，即修改的線性/多項式/指數比模型，其核心參數由Sentinel-2影像的綠光和藍光波段之間的對數比組成，以將影像的光譜數據與ICESat-2剖面深度資料相擬合。而在使用經過訓練的模型進行水深反演後，利用擬合最好的2張水深圖成果做加權平均以產製出最後水深圖成果，並使用獨立的ICESat-2光子點雲驗證資料來驗證由Sentinel-2加權平均得出的水深圖精度表現。在這6個島中使用2張最佳影像加權平均的三個模型成果，在0至15公尺水深，RMSE約在0.56 m-0.97 m之內。本研究得出的結論是，透過測高衛星的資料和光學衛星影像的結合可以產製水深圖，其水深精度，在0至5公尺深的水深範圍內大致符合電子航海圖（Electronic Navigational Chart）的CATZOC (the Category Zone of Confidence）中的A2和B類別，而6至14公尺水深精度，則符合CATZOC中的C類別。未來可以預見，ICESat-2將成為監測全世界沿海和淺水地區的有利工具，尤其是在沒有測深數據的地區。
;To derive shallow water bathymetry for coastal areas, a common approach is to deploy a scanning airborne bathymetric light detection and ranging (LiDAR) system or a shipborne echosounder for ground surveys. However, recent advancements in satellite remote sensing, including the Ice, Cloud and land Elevation Satellite-2 (ICESat-2) offer new tools for generating satellite derived bathymetry (SDB). The key payload onboard ICESat-2 is the Advanced Topographic Laser Altimeter System (ATLAS), a micro-pulse, photon-counting LiDAR system, simultaneously emitting six separate 532 nm beams at a nearly continuous 10 kHz pulse rate. However, despite its high resolution, the major limitation for bathymetry is that ICESat-2 only provides along-track height profiles, leaving observation gaps between the parallel ground tracks. Merging ICESat-2 observations with optical imagery offering multispectral raster, as demonstrated herein, offers an effective solution for deriving a full two-dimensional scene of water depth in light of the spectral attenuation behavior.
This study aims to combine ICESat-2 and Sentinel-2 optical imageries to derive shallow water bathymetry (depth less than 20 m) at six islands and reefs in the South China Sea. ICESat-2 ATL03 point clouds of georeferenced photons are first filtered to determine the seafloor elevation along the ground track. Results indicate a root-mean-square error (RMSE) of 0.20–0.47 m as compared with independent observations from an airborne LiDAR campaign. Next, three semi-empirical functions, namely the Modified Linear/Polynomial/Exponential Ratio Models with its kernel formed by the log ratio between Sentinel-2’s green and blue bands, are used to fit the spectral data with ICESat-2 height profiles. After water depth mapping using the trained model, independent ICESat-2 point clouds are used to validate the Sentinel-2 derived bathymetry. The RMSE of the three models using the weighted average from the best two images for these six islands are within 0.56 m–0.97 m in 0–15 meter deep. We also demonstrate that a synthesis of satellite laser altimetry and optical remote sensing can produce SDB results that potentially meet the requirement of category A2 and B in Zones of Confidence (ZOC) of the Electronic Navigational Chart (ENC) in 0–5 m deep, and satisfy category C in 6–14 meter deep. It is foreseen that ICESat-2 will be a helpful tool for mapping coastal and shallow waters around the world especially where bathymetric data are unavailable. |
