淺水多音束聲納資料之誤差檢核分析研究

Abstract

多音束測深在施測時必需依賴周邊儀器提供參數進行水深判定，因支援儀器繁雜，且水深測點的資料量龐大，無論是在施測當下的衛星定位或是資料後處理的參數設定，都容易造成難以修復的誤差。本文將對於各種參數影響造成的誤差量，利用交叉分析法來檢示不同誤差因素所產生的誤差分布，並嘗試予以修正。在定位誤差方面使用GPS單機定位與DGPS差分定位做對照，單機定位的誤差量大，且誤差值及位移方向會隨著時間而改變，也因此，定位誤差無法適用固定的誤差值去做修正。於水深25公尺處地形坡度超過1.3度以上，則GPS單機定位之精度並無法滿足IHO特級測深標準，必需使用DGPS定位解。在多音束音鼓安裝時若存在橫搖角偏差1°就會使得資料品質大幅下降，誤差分布有對稱性且呈現正負值相間的鋸齒狀。存在縱搖角偏差則會使誤差分布偏向正值，但在地形平坦處(水深25公尺)無太大的影響。檢核線與測線應該為正相交，檢核航線可通過測線上內側和外側音束位置，其交叉分析所得的結果才具參考意義，若檢核航線與測線不規則相交，則其交叉分析結果無法正確反應出全樣本的統計值。

Multi-beam echo sounders are normally equipped with kinds of Peripheral devices for the measurements. However due to the complexity of supporting devices, and massive amount of water depth data, a considerable amount of Irreversible artifacts will occur. This study uses Cross Over Check method to survey error distribution that caused by different means, and attempt to correct the discrepancies. In the problem of positioning error, GPS measurements have poor quality than the DGPS (differential GPS) and deviation of the errors shows time dependence. Therefore, there is no way to apply a constant correction value to fix the problem. Moreover, it is estimated that the GPS solutions can be used when the sounding area with the relief of the slope less than 1.3 degree. When the transducer is installed with more than 1 roll offset angle, bathymetry accuracy will easily fail to satisfy the IHO level when seabed is 25m deep. Pitch offset angle will cause the error shift towards positive value, but the occurrence is slight when seabed is shallow and flat. Checking the errors of every track can be realized with check line crossing with track, check line crossing with all locations of sound beam footprints. Direction of the cross over lines shall be perpendicular with the azimuth of survey tracks, such that check lines can be crossing inner and outer beams of the survey lines at almost equal proportion. Under this condition, the output value to judge the quality of bathymetry can be accepted statistically.