Digital Aerial Photogrammetry can Enhance Monitoring Efficiency of Urban Forest Changes in Hong Kong

Urban trees are characterized by high turnover rates and require the development of efficient tools to understand their spatial and temporal dynamics. While the official mapping agency in Hong Kong regularly collects aerial photos and provides them as open data, this study created multi-temporal three-dimensional models from the freely available dataset using digital aerial photogrammetry (DAP) and aimed to examine the changes in urban forests in 2010-2021. Despite the variety of camera configurations and flight settings over the years, DAP produced similar tree heights on roadside trees in Hong Kong compared to airborne LiDAR data. Besides providing a higher observation frequency and revealing more detailed variations in the temporal patterns, the DAP dataset could also accurately detect removed trees by analyzing the structural changes, including the loss of urban vegetation after typhoons. It creates opportunities for both retrospective and continuous monitoring of urban forest changes.