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The retrieval of two-wavelength GPR models based on log(1/R). One wavelength was selected around the green peak (540–570 nm) and the other around the red edge region (700–730 nm). (a,b), respectively, displays the R2 and RMSE values in the calibration, and (c,d) displays those in the validation. The unit of the RMSE is nmol/cm2.
