Overlapping seabed images collected at Looe Key, Florida, 2021

IMAGE ACQUISITION Images were collected in tiff (.tif) format in synchronized quintets by the SQUID-5 system while towed at a speed of approximately 3 knots behind the USGS vessel R/V Sallenger during USGS field activity 2021-633-FA, also known as 2021-301-FA. The image-collection frequency was 1 Hz, and GNSS positions were recorded at 10 Hz. Image-gain settings were adjusted as needed depending on water depth and lighting conditions. Aperture and shutter speed were consistent for each group of images at a study site. Image files were named programmatically with the last two digits of the camera's serial number, a 14-digit UTC datetime value formatted as yyyymmddhhmmss, thousandths of seconds, and the sequential image number for that camera. For example, 'CAM13-20210717163118_000-330.tif' indicates camera 13, July 17, 2021, 16:31:18.000 (UTC), collection event number 330 of the survey line.

SQUID-5 NAVIGATION PROCESSING Raw GNSS data received by the antenna mounted atop the SQUID-5 were recorded at 10 Hz by a dual-frequency survey-grade GNSS receiver, which was post-processed and differentially corrected against a local base station located 14 km from the survey site, in Novatel's GrafNav software (v. 8.80.2720; https://novatel.com/products/waypoint-post-processing-software/grafnav) to produce 10-Hz SQUID-5 trajectories. The SQUID-5 trajectories, in combination with precisely recorded image capture event times, are used to generate SQUID-5 GNSS antenna positions at the moment of each image capture in the accompanying navigation file (SQUID5_LKR_2021_Image_Locations.txt). The positions in the SQUID-5 navigation file represent the position of the SQUID-5 GNSS antenna reference point (ARP), not the position of the cameras or features imaged. To determine actual image positions, photogrammetric software such as Agisoft Metashape can be used to apply the lever arm offsets (in the camera frame of reference) from the GNSS antenna reference point to the camera lens. Using this method the following values for each camera were calculated for x,y,z in units of meters. Camera 13, 0.028 m(x), 0.016 m(y), 0.822 m(z); Camera 39, 0.274 m(x), -0.106 m(y), 0.912 m(z); Camera 75, 0.128 m(x), 0.562 m(y), 0.741 m(z); Camera 82, -0.015 m(x), -0.570 m(y), 0.765 m(z).

IMAGE EXIF SORTING Georeferencing, copyright, and other relevant information were added to the EXIF header of each image using Phil Harvey's ExifTool (v. 11.88; https://exiftool.org/). To extract the information from the image headers using ExifTool, the following command can be used (tested with ExifTool version 11.88): exiftool -csv -f -filename -GPSDateStamp -GPSTimeStamp -GPSLongitude -GPSLatitude -GPSAltitude -n -Artist -Credit -comment -keywords -Caption -Copyright -CopyrightNotice -Caption-Abstract -ImageDescription directoryname/*.tif > out.csv The -csv flag writes the information out in a comma-delimited format. The -n option formats the latitude, longitude, and altitude as signed decimal values. Note that the altitude is referenced to the NAD83 (2011) ellipsoid and are all negative for this dataset. CAUTION: Without the -n option, the command may return the absolute value of the altitude. Images are sorted by survey line number and stored within zip files to limit the file sizes.