A bathymetric compilation around the South Orkney Islands, 1987-2017

• Main Author: Abrahamsen, E. Povl
• Format: Dataset
• Language: English
• Published: UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation 2019
• Subjects: "EARTH SCIENCE","LAND SURFACE","TOPOGRAPHY"; "EARTH SCIENCE","OCEANS","BATHYMETRY/SEAFLOOR TOPOGRAPHY","BATHYMETRY"; "EARTH SCIENCE","OCEANS","BATHYMETRY/SEAFLOOR TOPOGRAPHY","SEAFLOOR TOPOGRAPHY"; "EARTH SCIENCE","OCEANS","BATHYMETRY/SEAFLOOR TOPOGRAPHY","WATER DEPTH"; "EARTH SCIENCE","OCEANS","BATHYMETRY/SEAFLOOR TOPOGRAPHY"; Bathymetry; Multibeam; South Orkney Islands; Orkney Passage; South Orkney Trough; Weddell Sea; Scotia Sea; Antarctic; Weddell; Alfred Wegener Institute; Antarc*; British Antarctic Survey
• Online Access: https://dx.doi.org/10.5285/1158843c-d238-439a-ad42-9645f37ac582; https://data.bas.ac.uk/full-record.php?id=GB/NERC/BAS/PDC/01196

We present a new bathymetric compilation around the South Orkney Islands here defined by the following bounding box: 47 to 37 W, 63 to 59 S. This bathymetry grid was compiled from a variety of multibeam swath bathymetry data acquired during 46 different cruises (see lineage). The data is available as a grid of approximately 100 m resolution in a GMT-compatible (2-D) NetCDF format using geographic coordinates on the WGS84 datum. Three versions of the grid are available: the first one shows only swath bathymetry data while the second and third have been merged with the global compilations from the General Bathymetric Chart of the Oceans (GEBCO), GEBCO_2014 (version 20150318) and GEBCO_2019, respectively. Funding was provided by the NERC grants NE/K012843/1 and NE/N018095/1 as well as national capability : The bathymetry map was compiled from a variety of different data sources. The primary data are multibeam swath bathymetry collected from scientific cruises undertaken by British Antarctic Survey (BAS), Alfred Wegener Institute (AWI), or acquired during RV Nathaniel B. Palmer, RRS James Cook and ESPS RV Hesperides expeditions. The complete list of the cruises used for the compilation is available in the supplementary documents. The multibeam data are merged with GEBCO_2014 (version 20150318) and GEBCO_2019 (doi: 10.5285/836f016a-33be-6ddc-e053-6c86abc0788e). Both of these grids are available from https://www.gebco.net The grid with bathymetry data only was created using the mbgrid program from MB-system version 5.5.2336 (https://www.mbari.org/products/research-software/mb-system/), using the parameters "-E100/100/meters -R-47/-37/-63/-59 -G3 -A2 -T25 -W4 -N -M -C5/1 -P1 -F1 -V". This uses a Gaussian weighted mean filter. Values are interpolated using a spline (tension 25) up to five grid cells from the nearest data point only if additional data are found in two opposite directions. To merge with the global grids, the resulting grid is then loaded into Matlab R2019a. A subset of the GEBCO_2014 grid at 30 arcseconds or GEBCO_2019 at 15 arcseconds resolution is loaded into Matlab and interpolated onto the same grid. The grids are then subtracted, and the resulting grid is expanded by one grid cell in all directions, with the new cells filed with zeros. A list of the resulting values (excluding those that do not have any multibeam data) is exported to a temporary text file. This file is then run through mbgrid with parameters "-G3 -A2 -T50 -C9999/3 -W50 -V", resulting in a grid of differences between the multibeam and background data, with all data gaps filled with a spline. This offset is then subtracted from the background data, and the resulting values substituted into the gaps in the multibeam grid. The Matlab codes are available in the supplementary documents. : Quality control was undertaken manually to remove any outliers in the source datasets. This involved cleaning much of the raw data using the "mbedit" program from MB-system. For data processed in this way, edit files have been submitted to the Polar Data Centre. In source files that were ingested as text files, outliers were manually removed. To verify the accuracy of the resulting dataset, bottom depths were compared against CTD casts on cruise JR16005, excluding casts that were performed when the ship was moving and those where the CTD did not get within altimeter range of the bottom. Bottom depths were calculated from the CTD altimeter when near the bottom, and these were compared against the gridded dataset. On 81 casts, the median difference between the CTD-derived depths and gridded depths was 4.1 m with a standard deviation of 18.9 m. This compares favourably with GEBCO_2014, which had a median offset of 4.6 m with a standard deviation of 73.4 m, and GEBCO_2019, which has a median offset of 0.5 m but a standard deviation of 89.4 m.