Evaluating simple proxy measures for estimating depth of the ~ 1900 nm water absorption feature from hyperspectral data acquired under natural illumination

Abstract Direct measurement of depth of the ~ 1900 nm feature (F 1900 ) caused by molecular water is not possible from hyperspectral data acquired under natural illumination. Proxy measures for depth were evaluated to estimate the true depth of F 1900 from such data. Suitable proxy measures were identified using data acquired by a high-resolution spectrometer (under artificial and natural illumination) and by a hyperspectral imaging sensor (under artificial illumination), with different amounts of simulated atmospheric noise. The best performing proxy measures were used to estimate depth from hyperspectral imagery acquired in the field under natural illumination. Proxy measures comprised a ratio of reflectance, continuum-removed depth at a single wavelength and continuum-removed depth integrated over several wavelengths. For high resolution spectra, a ratio of reflectance at 2017 nm and 1967 nm (Ratio 2017/1967 ) and the continuum-removed depth at 1967 nm (CRD 1967 ) were strongly correlated with depth R 2  = 0.98–0.99 and R 2  = 0.95–0.96, respectively. For hyperspectral imagery acquired in the laboratory, Ratio 2017/1967 and CRD 1967 were strongly correlated with depth across all levels of atmospheric noise (R 2  = 0.94–0.98 and 0.94–0.97, respectively). Proxy measures using integrated depth performed relatively poorly (R 2  = 0.65–0.86). In independent tests, depth of F 1900 was consistently overestimated by CRD 1967 but not by Ratio 2017/1967 . Validation of field imagery was done by comparing depth predicted by the proxy measures with depth measured from laboratory imagery of collocated samples from the field. Ratio 2017/1967 derived from a polynomial fit to the data between 1961 and 2134 nm (RatioP 2017/1967 ) most closely approximated measured depth. Ratio 2017/1967 , derived from the original data either under or overestimated depth. Measures of continuum-removed depth generated from the original data (CRD 1967 ) and from a polynomial fit to the original data (CRDP 1967 ) overestimated the depth of F 1900 . This study showed that the depth of F 1900 can be predicted from data acquired under natural illumination however, the choice of proxy measure can have a significant impact upon quantitative estimates.