Fig. 4. (a) Principal component similarity maps determined for visible and near infrared (VIS-NIR) spectra of the multiple-field dataset, showing Groups 5 (0.0–0.060 kg kg^–1), 10 (0.061–0.120 kg kg^–1), 15 (0.121–0.180 kg kg^–1), 20 (0.181–0.240 kg kg^–1), and 25 (0.241–0.400 kg kg^–1), and (b) spectral patterns SP1 and SP2 corresponding to the principal components PC1 and PC2, respectively.