Multi-Angular Reflectance Anisotropy Observed from UAV Multispectral Imagery

2026-06-08

AI Summary

This study addresses geometrically induced radiometric inconsistencies in low-altitude UAV multispectral imagery, which arise from wide fields of view and multi-angle observations. The authors propose a geometry-aware method to systematically extract multi-angle reflectance measurements and corresponding viewing geometry for the same ground target from UAV multispectral data for the first time. By refining camera interior and exterior parameters via structure-from-motion (SfM), homogeneous regions identified in orthomosaics are back-projected onto original multi-view sub-images, enabling joint retrieval of ten-band reflectance values and observation angles, and facilitating anisotropic reflectance visualization. Experimental results demonstrate that reflectance extremes for grass targets differ by 119%–137% in the red-edge and near-infrared bands, providing strong evidence of the significant impact of observation geometry on radiometric consistency.

Abstract

UAV multispectral imagery naturally contains multi-angular observations due to low flight altitude and wide field-of-view imaging, which may introduce geometry-driven radiometric variability. This study proposes a geometry-aware multi-angular observation extraction workflow to quantify observation-geometry effects from a BRDF perspective. Specifically, camera intrinsics and extrinsics are refined via structure-from-motion (SFM), and homogeneous regions annotated on an orthomosaic are reprojected onto multiple raw sub-images acquired from different viewpoints. This enables joint extraction of multi-band reflectance and observation geometry parameters for the same ground targets under varying viewing directions. The extracted observations are further analyzed using band-wise polar visualization in the (VZA, RAA) domain. Results on a grassland target show clear reflectance anisotropy across ten bands, with red-edge and nearinfrared bands exhibiting 119-137% variability between maximum and minimum reflectance, indicating non-negligible observation-geometry effects on radiometric consistency.

Problem

• multi-angular reflectance
• observation geometry
• radiometric variability
• BRDF
• UAV multispectral imagery

Innovation

• multi-angular reflectance
• UAV multispectral imagery
• BRDF
• structure-from-motion (SfM)
• reflectance anisotropy