Variable indicators for optimum wavelength selection in diffuse reflectance spectroscopy of soils
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MC Sarathjith, Bhabani Sankar Das, Suhas Wani, Kanwar Lal Sahrawat. (5/5/2016). Variable indicators for optimum wavelength selection in diffuse reflectance spectroscopy of soils. Geoderma, 267, pp. 1-9.
Abstract
Diffuse reflectance spectroscopy (DRS) operating in 350–2500 nm wave length range is fast emerging as a rapid and
non-invasive technique for analyzing multiple soil attributes. Because the spectral reflectance values in this range of
wavelengths are highly co-linear, it is important to select relevant spectral information from the reflectance spectra
to build a robust spectral algorithm. The objective of this study is to examine the utility of different variable indicators
such as partial least squares regression (PLSR) coefficients (β), variable influence on projection, squared residual
(SqRes), correlation coefficient (r), biweightmidcorrelation (bicor), mutual information based adjacency value (AMI),
signal-to-noise ratio (StN), covariance procedures (CovProc) and their combinations in conjunction with an ordered
predictor selection (OPS) approach for selecting optimum number of spectral variables (NSV)which could improve
DRS model performance. The approach was tested with the PLSR models of pH, organic carbon, extractable iron (Fe),
sand and clay contents and geometric mean diameter in Vertisols and Alfisols. The prediction accuracy of best
models selected via OPS approach was found to be superior to full-spectrum (NSV = 2048) model for all the soil
attributes. The percent decrease in RMSE value was found to be highest for Fe (14%, NSV=79) in Alfisols followed
by pH (9%, NSV=660) in Vertisols while it varied between 3 and 8% for other soil attributes. Although the results
were not conclusive in favor of one specific variable indicator, the CovProc and bicorwere found to be more appropriate
for accurate and moderate DRS models in this study, respectively. The overall results of this study advocate the
use of OPS approach with variable indicators and their combinations as a promising strategy to develop simple and
effective DRS models for soils.