Digital surface model representing Head of the Meadow Beach, Truro on March 10, 2022

The Digital Surface Model was created in Agisoft Metashape v. 1.8.1 using the following general steps (see Over and others, 2021 for a more detailed methodology explanation): 1. The project was created and imagery (located in larger work citation 2022015FA_Truro_f1.zip) and positions (located in larger work citation 2022015FA_Truro_f1_photolocations.csv) were imported using the NAD83(2011)/UTM19N and NAVD88 datums. An 0.15 m GNSS/IPS offset in the z direction was used to represent the approximate offset of the GNSS mount and camera on the Helikite. 2. Photos were aligned at a low accuracy so that GCPs could be automatically detected in the point cloud. Eighteen GCPs (2022015FA_Truro_nav_GCPs.csv located in the larger work citation) were found in the images and then added to the project in the reference systems NAD83(2011)/UTM Zone 19N and NAVD88. Accuracies for the GCPs were set to 0.02 m and accuracies for the camera positions were set to 0.10 m, but the positions were only used as check points and to speed up alignment, as they were unchecked in the software. The photos were re-aligned with high accuracy (the pixels were not subsampled increase processing speed) using a keypoint limit of 40,000 and unlimited tie points. 3. The alignment process matched pixels between images to create point clouds and put the imagery into a relative spatial context using the photo positions and GCPs. Then, the Metashape software refined and optimized the camera positions and lens model using least squares with gradual selection parameters of Ru = 15, Pa = 4, and Re = 0.35 to minimize the reprojection error or the distance between the measured points and the software-created points. 4. A dense point cloud was generated using the high-quality setting (images were not subsampled) and a low-frequency filtering algorithm. The dense point cloud was then edited to remove noise by filtering by point confidence before generating an interpolated digital elevation model (the software calls all models elevation models, but a surface model includes vegetation/canopy returns; a terrain model would be only bare earth returns). 4. The DSM product was exported at 25 cm in NAD83(2011) UTM Zone 19N and NAVD88 (m).

The DSM was opened in QGIS and a shapefile was created (not included in this data release) that excluded areas of interpolation and obviously erroneous elevation points on the outer edge. The DSM was then clipped to this shapefile and turned into a cloud-optimized GeoTIFF (COG) using gdal_translate with the following command: for %i in (.\*.tif) do gdal_translate %i .\cog\%~ni_cog.tif -of COG -stats -co BLOCKSIZE=256 -co COMPRESS=DEFLATE -co PREDICTOR=YES -co NUM_THREADS=ALL_CPUS -co BIGTIFF=YES (v. 3.1.4 accessed October 20, 2020 https://gdal.org/), where i is the name of each GeoTIFF section.