Zhang et al. (2011) presented a novel approach by constructing a spectral knowledge base
(SKB) of diseased winter wheat plants, which takes the airborne images as a medium and links
the disease severity with band reflectance from environment and disaster reduction small
satellite images (HJ-CCD) accordingly. Through a matching process with a SKB, we estimated
the disease severity with a disease index (DI) and degrees of disease severity. The proposed
approach was validated against both simulated data and field surveyed data. Estimates of DI
(%) from simulated data were more accurate, with a coefficient of determination (R 2) of 0.9 and
normalized root mean square error (NRMSE) of 0.2. The overall accuracy of classification
reached 0.8, with a kappa coefficient of 0.7. Validation of the estimates against field
measurements showed that there were some errors in the DI value with the NRMSE close to
0.5. The result of the classification was more encouraging with an overall accuracy of 0.77 and
a kappa coefficient of 0.58. For the matching process, Mahalanobis distance performed better
than the spectral angle (SA) in all analyses in this study.