3D Reconstruction and Approximation of Vegetation Geometry for Modeling of Within-canopy Flows

Aquatic vegetation can shelter coastlines from waves and currents, sometimes resulting in accretion of fine sediments. We developed a photogrammetric technique for estimating the key geometric vegetation parameters that are required for modeling of within-canopy flows. Accurate estimates of vegetation geometry and density are essential to refine hydrodynamic models, but accurate, convenient, and time-efficient methodologies for measuring complex canopy geometries have been lacking. The novel approach presented here builds on recent progress in photogrammetry and computer vision. We analyzed the geometry of aerial mangrove roots, called pneumatophores, in Vietnam’s Mekong River Delta. Although comparatively thin, pneumatophores are more numerous than mangrove trunks, and thus influence near bed flow and sediment transport. Quadrats (1 m²) were placed at low tide among pneumatophores. Roots were counted and measured for height and diameter. Photos were taken from multiple angles around each quadrat. Relative camera locations and orientations were estimated from key features identified in multiple images using open-source software (VisualSfM). Next, a dense 3D point cloud was produced. Finally, algorithms were developed for automated estimation of pneumatophore geometry from the 3D point cloud. We found good agreement between hand-measured and photogrammetric estimates of key geometric parameters, including mean stem diameter, total number of stems, and frontal area density. These methods can reduce time spent measuring in the field, thereby enabling future studies to refine models of water flows and sediment transport within heterogenous vegetation canopies.