Optimizing UAV Hyperspectral Imaging for Urban Tree Chlorophyll and Leaf Area Index Retrieval

Uncrewed aerial vehicle (UAV) based hyperspectral imaging offers a flexible method for monitoring urban trees. However, its effect on estimating biochemical and biophysical parameters is still unknown. This article examines how spatial and spectral resolution, solar zenith angle (SZA), and diffuse solar irradiance (SKYL) affect chlorophyll content (C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub>) and leaf area index (LAI) estimation using narrow-band indices (NBIs) through three-dimensional radiative simulations. The results show that spatial resolution minimally affects C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> estimation but significantly impacts LAI, with finer resolutions improving correlation with NBIs. In contrast, spectral resolution has little effect on LAI but greatly influences C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub>, with a 2-nm resolution providing stronger correlations, while resolutions coarser than 6 nm are less sensitive. The C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> estimation prefers oblique SZAs, while LAI favors nadir SZAs. SKYL has little effect on C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> and minor impact on LAI. Sunlit pixels outperform shaded ones for C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> estimation, even at 2-m resolution, while entire-crown pixels show the highest LAI correlation. Different NBI strategies significantly affect LAI estimation but not C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub>. A consistent conclusion emerges from the analysis of correlations between UAV hyperspectral imagery, with varying spatial and spectral resolutions, and corresponding C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> field measurements. This suggests that the knowledge revealed by the radiative transfer model is applicable to real-world conditions and improves understanding of natural processes without direct measurements.This article enhances the understanding of the influence of observation configurations on C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab</sub> and LAI estimation, offering insights to optimize UAV-based hyperspectral imaging and guide future satellite sensor development for tree monitoring.