Volumetric Change Detection with SfM Photogrammetry from Rapid-Response Aerial Imagery After Hurricane Helene

Civil Air Patrol (CAP) imagery, often the first post-disaster observation, is designed for 2D interpretation rather than multi-temporal volumetric measurement. We build post-storm digital surface and terrain models from uncontrolled CAP imagery with structure-from-motion in western North Carolina’s open Swannanoa Valley and forested Hickory Nut Gorge. Surfaces are co-registered to pre-event LiDAR using roads as pseudo ground control, corrected for terrain-correlated bias, and thresholded with a spatially variable, calibrated Level of Detection. Before bias correction, bare-earth surfaces match withheld post-event LiDAR on stable terrain to 0.68 and 0.82 m normalized median absolute deviation. Volumes are 2017-to-2024 change, lower bounds in coverage, and conditional on the bias model, residual screen, and detection window. Corrected valley surfaces record 125,000 m3 of erosion and 123,000 m3 of deposition, and a sampled image check dates about 13% of that change to the sortie. Under the same limit, the uncorrected surface reports five times the deposition, and the alternative bias model gives gross terms 3.6 to 4.2 times larger. The gorge at Chimney Rock recorded 76,000 m3 of erosion and 84,000 m3 of positive change, just under half dating to the sortie, including land deposition partly cleared before the post-event LiDAR flew. CAP-derived surfaces can capture transient storage that later surveys cannot.