Piepenburg, Dieter (2016): Seabed photographs taken along OFOS profiles during POLARSTERN cruise PS96 (ANT-XXXI/2 FROSN) [dataset publication series]. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA, https://doi.org/10.1594/PANGAEA.862097

Within the context of the overall ecological working programme Dynamics of Antarctic Marine Shelf Ecosystems (DynAMo) of the PS96 (ANT-XXXI/2) cruise of RV "Polarstern" to the Weddell Sea (Dec 2015 to Feb 2016), seabed imaging surveys were carried out along drift profiles by means of the Ocean Floor Observation System (OFOS) of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) Bremerhaven.

The setup and mode of deployment of the OFOS was similar to that described by Bergmann and Klages (2012, doi:10.1016/j.marpolbul.2012.09.018). OFOS is a surface-powered gear equipped with two downward-looking cameras installed side-by-side: one high-resolution, wide-angle still camera (CANON® EOS 5D Mark III; lens: Canon EF 24 f/1.4L II, f stop: 13, exposure time: 1/125 sec; in-air view angles: 74° (horizontal), 53° (vertical), 84° (diagonal); image size: 5760 x 3840 px = 21 MPix; front of pressure resistant camera housing consisting of plexiglass dome port) and one high-definition color video camera (SONY® FCB-H11).

The system was vertically lowered over the stern of the ship with a broadband fibre-optic cable, until it hovers approximately 1.5 m above the seabed. It was then towed after the slowly sailing ship at a speed of approximately 0.5 kn (0.25 m/s). The ship's Global Acoustic Positioning System (GAPS), combining Ultra Short Base Line (USBL), Inertial Navigation System (INS) and satellite-based Global Positioning System (GPS) technologies, was used to gain highly precise underwater position data of the OFOS.

During the profile, OFOS was kept hanging at the preferred height above the seafloor by means of the live video feed and occasional minor cable-length adjustments with the winch to compensate small-scale bathymetric variations in seabed morphology. Information on water depth and height above the seafloor were continuously recorded by means of OFOS-mounted sensors (GAPS transponder, Tritech altimeter).

Three lasers, which are placed beside the still camera, emit parallel beams and project red light points, arranged as an equilateral triangle with a side length of 50 cm, in each photo, thus providing a scale that can be used to calculate the seabed area depicted in each image and/or measure the size of organisms or seabed features visible in the image. In addition, the seabed area depicted was estimated using altimeter-derived height above seafloor and optical characteristics of the OFOS still camera.

In automatic mode, a seabed photo, depicting an area of approximately 3.45 m² (= 2.3 m x 1.5 m; with variations depending on the actual height above ground), was taken every 30 seconds to obtain series of "TIMER" stills distributed at regular distances along the profiles that vary in length depending on duration of the cast. At a ship speed of 0.5 kn, the average distance between seabed images was approximately 5 m. Additional "HOTKEY" photos were taken from interesting objects (organisms, seabed features, such as putative iceberg scours) when they appeared in the live video feed (which was also recorded, in addition to the stills, for documentation and possible later analysis).

If any image from this collection is used, please cite the reference as given above.

According to the manufacturers' specifications, the GPS receiver of Polarstern (Trimble SPS461) has an accuracy of ?better than 5 m 3DRMS? (without differential signal that was not available during the cruise), and the GAPS system has an accuracy of 0.17% of slant ranges, which correlate best with cable lengths, which in turn were very close to water depths at the operational conditions of OFOS deployment during the cruise (e.g., small towing speed, so that OFOS was constantly hanging almost directly below the stern of the ship during the entire cast). However, the actual accuracy is known to be always less than the manufacturer's specifications because shipboard hydroacoustic systems are highly influenced by a variety of factors (e.g., transducer installation and calibration, ship's movements and noise, acoustic water-column characteristics). Therefore, the actual accuracies of Polarstern's underwater positioning systems were found to be 1 to 1.5% of slant range for 95% of the data, based on the ship's data post-processing/filtering/validation history. This suggests that for the depth range of OFOS stations during PS96 (about 200 to 750 m), a range of 0.2 to 8 m should be added to Polarstern's GPS accuracy. The spatial offsets between positioning devices on board and in the water were corrected automatically. In conclusion, the overall accuracy of the spatial positions of OFOS-derived seabed images can be estimated to fall into the range of 3 to 10 m, depending on water depths. However, the relative precision of the determination of the spatial intervals between the images taken along a drift transect is assumed to be one order of magnitude higher, i.e., these distances can be assessed with a precision on a m-scale. Accordingly, the coordinates of the seabed images are given as decimal degrees (DD) expressed to five decimal places, corresponding to a spatial resolution in the range of 0.3 (for longitudes at latitudes of about 70°S) to 1.1 m (for latitudes).