Applicability Evaluation of BRDF Inversion Models at High-Resolution Scale Based on UAV Multiangle Multispectral Images

With the development of uncrewed aerial vehicle (UAV) multiangle and multispectral observation, whether bi-directional reflectance distribution function (BRDF) models are applicable to high spatial resolution has become the focus of researchers. However, previous studies usually focus on soil-vegetation BRDF models, limited to certain natural types. In this study, UAV multispectral and multiangle observations were conducted over both natural and artificial targets, to investigate the heterogeneity and similarity among different targets. Five representative BRDF models were selected for inversion, including the Walthall model, the seven-parameter model with double-peak, the RossThick-LiSparseReciprocal-Chen (RTLSR_C) kernel-driven model, the Enhanced Rahman–Pinty–Verstraete (ERPV) model, and the Hemisphere Harmonics (HSH) basis model. To compare the ability of these models in capturing the optical scattering characteristics, observation data from different observation planes were used for fitting. The results show that: (1) The five selected BRDF models generally remain applicable at the UAV scale based on the fitting RMSEs. (2) When only the cross principal plane (CPP) data are used, the RTLSR_C model achieves the best inversion performance. When only the principal plane (PP) data are used, the ERPV model performs best. (3) The multispectral characteristics of the green camouflage net are most similar to that of the grassland.