Coastal Interferometric Swath Bathymetry Data Collected in 2015 from the Chandeleur Islands, Louisiana: 2015_Chand_IFB_5m_NAD83_NAVD88_GEOID09_XYZ

Swath Bathymetry Acquisition: The interferometric swath bathymetry data were collected aboard the R/V Sallenger in September 2015 using the SEA SWATHplus-H 468 kHz interferometric sonar system. The transducer system was fastened to a static pole mount deployed over the starboard side gunnel. The Novatel GPS antennas were mounted atop the cabin. Boat position and motion data were recorded in real-time using a CodaOctopus F190R+ wetpod IMU. The IMU was mounted underwater between the transducer heads to minimize lever arm geometry errors between observed depths and vessel motion. Real-time corrected positions were acquired via Marinestar HP DGPS subscription service from Fugro, Inc. Marinestar HP position correction data and motion data from the IMU were integrated with interferometric soundings using SWATHplus software, version 3.10.2.0, with positional and calibration offsets pre-defined by a session file (.sxs), allowing for real-time-corrected depths. Before deployment, all equipment offsets were measured. During the survey, all swath tracklines were recorded in SWATHplus raw data format (.sxr). A Valeport Mini Sound Velocity Sensor (miniSVS) was attached to the transducer mount and collected continuous speed of sound (SOS) measurements at the depth of the transducers. These values were directly read and incorporated into the SWATHplus raw data format during acquisition providing a real-time speed of sound at the transducer while underway. In addition, a separate Mini Sound Velocity Profiler (Valeport miniSVP) was used to collect speed of sound profiles (water surface to seafloor) approximately every two hours throughout the survey as well as other areas of question such as rapid transitions in water depth.

Swath Bathymetry Processing: Data were collected with SWATHplus-H and Bathyswath 3.10.2, an acquisition and initial processing program. Preliminary roll calibration trackline data were collected and processed using Bathyswath Swath Processor version 3.10.2.0 and Bathyswath Grid Processor software version 3.10.0.0. Instrument offsets and calibration values were input into the session file (.sxs) and the raw data files (.sxr) were then processed using the updated system configuration file containing roll calibration values, measured equipment offsets, acquisition parameters, navigation and motion from the F190R+, SOS at the sonar head, and SVP cast data. Any calibration offsets and acoustic filtering utilized in Bathyswath was then applied to each raw data file to create a processed data file (.sxp), and then imported into advanced sounding data processing software CARIS HIPS and SIPS (CARIS) version 9.1. The initial real-time processing datum for the swath and backscatter data was WGS84, which is the acquisition datum for Marinestar HP position and navigation data. Within CARIS, outliers for the navigation data and processed soundings were removed using program filters. Any remaining data outliers were then removed manually. A CARIS BASE surface with associated CUBE sample surface was created from the edited soundings. A CUBE hypothesis is the estimated value of a grid node representing all the soundings within a chosen resolution or grid-cell size (for example, 5 m) weighted by uncertainty and proximity, giving the final value as a "sample" of the data within the specific grid cell. This algorithm allows for multiple grid-node hypotheses to be verified or overridden by the user while maximizing processing efficiency. A final visual inspection of the dataset in addition to Quality Control and Quality Assurance (QA/QC) procedures were implemented to ensure all outliers and erroneous data points had been removed from the final BASE surface. In order to provide the dataset in a non-proprietary format, the final XYZ data were exported as an ASCII comma-separated file.

Datum Transformation: The text file 2015_Chand_IFB_5m_NAD83_NAVD88_GEOID09.txt was converted using VDatum version 3.5 from WGS84 to the North American Datum of 1983 (NAD83) reference frame and the North American Vertical Datum of 1988 (NAVD88) orthometric height using the National Geodetic Survey (NGS) geoid model of 2009 (GEOID09).

Added keywords section with USGS persistent identifier as theme keyword.