Viewing-geometry effects on UAV-based tree height retrieval using BRDF-derived vegetation indices

Tree height is a key descriptor of forest canopy structure, yet a single nadir view captures only part of the directional structural information contained in canopy reflectance. This study investigated whether viewing geometry provides directional information useful for UAV-based optical tree-height retrieval in a Larix principis-rupprechtii plantation. Multi-angular reflectance observations extracted from overlapping UAV multispectral images were used to fit the RossThick–LiSparseR (RTLSR) kernel-driven bidirectional reflectance distribution function (BRDF) model. The fitted model was then used to reconstruct directional reflectance at 27 prescribed viewing geometries along the solar principal plane, while UAV-LiDAR data provided grid-level reference tree heights. Nine vegetation indices were derived and evaluated using linear regression, random forest, and gradient boosting regressor under single- and multi-angle schemes. DVI, EVI, GDVI, TVI, and GOSAVI exhibited consistent angle-dependent sensitivity to tree height, with their strongest correlations and lowest retrieval errors generally occurring within the intermediate forward-viewing sector (30°–45°) in the BRDF-derived dataset. For example, the absolute correlation between GDVI and tree height increased from 0.429 at nadir to 0.507 at 40°. Among the tested index–scheme–model combinations, the lowest validation error was obtained using GDVI under the Forward + 0° scheme with random forest, reducing RMSE from 2.19 m at nadir to 1.79 m (RMSE% = 10.25 %). However, the relative performance of the tested angular schemes varied among models, indicating that angular configuration and retrieval algorithm jointly affected performance, while increasing the number of angular features alone did not consistently reduce retrieval error. Overall, the findings indicate that viewing geometry can provide useful directional information for within-site optical tree-height retrieval and that intermediate forward-viewing information may complement nadir observations.