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

摘要

Direct measurement of depth of the similar to 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 (CRD1967) 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 CRD1967 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 CRD1967 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 (Ratio(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 (CRD1967) and from a polynomial fit to the original data (CRDP1967) 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. (C) 2015 The Author. Published by Elsevier Inc.