DTM and orthomosaic of a shallow water area near Allacciante - Valgrande channels (Venice Lagoon, Italy) - BRAHMS project

DTM and orthomosaic model of a 6-by-12-m shallow water area (average depth about 1.5 m) on a subidal flat near Allacciante and Valgrande channels, Lagoon of Venice (IT), generated by SfM photogrammetric approach. Photos were acquired on 11 May 2022 by a snorkel diver holding a hydrodynamic board equipped with 4 synchronized action cameras, 50-90 cm from the seafloor. Two pairs of cameras were fixed to the board 70 cm apart, each pair consisting of one camera facing nadir and one inclined approx. 45°. Cameras were set in time-lapse mode (2 fps, wide FoV, ISO 100-400). To cover the area, the diver swimmed back and forth, producing a pattern mostly composed of 11 longitudinal swaths partially overlapping. Reel lines were preliminarily deployed on the seafloor as a guide, together with 5 photogrammetric plates (33x33 cm). For each of 4 vertices and 5 plates, lat/long coordinates were acquired with Trimble SPS585 GNSS receiver (RTK corrections) and z-value as water depth measured with RBR solo3 depth logger. Analyses were perfomed with Agisoft Metashape. Sparse point cloud reconstruction was carried out through incremental alignment procedure, with a total of 2668 photos alligned. DTM (in m, water surface during depth measurements being zero elevation) was created with dense cloud basis and WGS84/UTM 33N projection. Orthomosaic was created based on DTM and same coordinate system. Scaling and georeferentiation were based on 9 ground control points (4 vertices and 5 plates). The resolution of DTM and orthomosaic are 2.01 mm/pix and 0.503 mm/pix, respectively.