ESTIMATING TRANSPARENCY IN TYPICAL BAY USING RGB SPECTRAL BANDS
Satellite remote sensing-based transparency(as in the Secchi depth(SD))inversion methods have been widely applied for oceans,offshore areas,and lakes.However,eutrophic estuaries,harbors,and other water bodies are relatively small in water area with significant land impact,and environmental monitoring in these waters requires timeliness.Due to the restrictions from temporal and spatial resolution,cloud shadowing,atmospheric correction,and other factors,the application in such small water bodies are limited.Therefore,it is urgent to establish an efficient and convenient transparency inversion method for these areas to complement satellite remote sensing methods,which calls for a robust SD inversion specific to these areas.This study attempts to construct a transparency inversion method for Xiangshan Bay,Zhejiang,East China,based on the red(R),green(G),blue(B)three bands using a conventional optical camera co-worked with smartphone HydroColor app and a drone,focusing on the red(R),green(G),and blue(B)bands.We observed that the drone and the app's red-to-blue(R/B)and red-to-green(R/G)ratios had a strong negative correlation with SD,and the correlation coefficients R was between-0.88 and-0.93(n=16,P<0.001).A transparency inversion model was established based on the results of correlation analysis and the accuracy of the model was evaluated based on an independent database.After the SD inversion model was calibrated,its accuracy was assessed using an independent dataset.Results show that the exponential inversion model is superior to the linear empirical model;the R/G inversion model is superior to the R/B model;and the HydroColor-based inversion model is superior to the drone-based inversion model.Based on these results,optimal SD inversion models were constructed for drone R/G and HydroColor-R/G determination.The average relative error and root mean square error of the drone model were 29%and 0.3 m,while those of the HydroColor model were 21.9%and 0.27 m,respectively.The above results indicate that both drones and smartphone apps can be used to obtain RGB data for SD inversion in Xiangshan Bay.The establishment of this method provides a new technical support for rapid and convenient water quality monitoring and red tide prevention and control in rivers,bays,and similar water bodies.
transparencyRGBremote sensing inversiondronesmartphone appXiangshan Bay