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 ~1900nm feature (F1900) 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 F1900 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 2017nm and 1967nm (Ratio2017/1967) and the continuum-removed depth at 1967nm (CRD1967) were strongly correlated with depth R2=0.98–0.99 and R2=0.95–0.96, respectively. For hyperspectral imagery acquired in the laboratory, Ratio2017/1967 and CRD1967 were strongly correlated with depth across all levels of atmospheric noise (R2=0.94–0.98 and 0.94–0.97, respectively). Proxy measures using integrated depth performed relatively poorly (R2=0.65–0.86). In independent tests, depth of F1900 was consistently overestimated by CRD1967 but not by Ratio2017/1967. 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. Ratio2017/1967 derived from a polynomial fit to the data between 1961 and 2134nm (RatioP2017/1967) most closely approximated measured depth. Ratio2017/1967, derived from the original data either under or overestimated depth. Measures of continuum-removed depth generated from the original data (CRD1967) and from a polynomial fit to the original data (CRDP1967) overestimated the depth of F1900. This study showed that the depth of F1900 can be predicted from data acquired under natural illumination however, the choice of proxy measure can have a significant impact upon quantitative estimates.