50-Meter Digital Elevation Model of Coastal Bathymetry Collected in 2011 from the Chandeleur Islands, Louisiana (U.S. Geological Survey Field Activity Numbers 11BIM01 and 11BIM02)

GPS Acquisition: A GPS base station was erected at a temporarily installed USGS benchmark (TMRK) located on the sound side of the Chandeleur Islands within about 15 km of the farthest single-beam trackline. GPS receivers recorded the 12-channel full-carrier-phase positioning signals (L1/L2) from satellites via the Thales choke-ring antenna. This GPS instrument combination was duplicated on the survey vessel (rover). The base receiver and the rover receiver record their positions concurrently at 1-second (s) recording intervals throughout the survey.

Single-Beam Bathymetry Acquisition: The single-beam bathymetric data were collected aboard the 22-foot R/V TwinVee. Boat motion was recorded at 50-millisecond (ms) intervals using a Teledyne TSS Dynamic Motion Sensor (TSS DMS-05). HYPACK version 10, a marine surveying, positioning, and navigation software package, managed the planned-transect information and provided real-time navigation, steering, correction, data quality, and instrumentation-status information to the boat operator. Depth soundings were recorded at 50-ms intervals using a Marimatech E-SEA-206 echosounder system with dual 208-kilohertz (kHz) transducers. Data from the GPS receiver, motion sensor, and fathometer were recorded in real time and merged into a single raw data file (.raw) in HYPACK, with each device string referenced by a device identification code and time stamped to UTC. Sound velocity measurements were collected using an Applied Microsystems Smart Sound Velocity Profiler (SVP). The instrument was cast overboard periodically throughout the survey to observe changes in water column speed of sound (SOS).

Swath Bathymetry Acquisition: The interferometric swath bathymetry data were collected aboard the R/V SurveyCat using a SEA SWATHplus-M 234 kHz interferometric sonar system mounted on a sled attached to a rail system fastened between the catamaran hulls allowing the instrument to align directly below the GPS antennae in order to minimize geometry errors. Boat position and motion data were recorded in real time using a CodaOctopus F190R wetpod inertial measurement unit (IMU) mounted underwater between the transducer heads to minimize lever arm geometry errors between the observed depths and associated vessel motion. Real-time corrected positions were acquired via the use of the OmniSTAR HP (High-Precision differential global navigation satellite system) satellite constellation subscription. OmniSTAR HP position correction data and motion data from the IMU were integrated with interferometric soundings in the SWATHplus software package with positional and calibration offsets predefined by a session file (.sxs), allowing for real-time-corrected depths. Prior to deployment, all equipment offsets were surveyed in dry dock with the use of a laser total station. During the survey all swath tracklines were recorded in SEA raw data format (.sxr). A Valeport Mini Sound Velocity Sensor (SVS) was attached to the transducer mount and collected continuous SOS measurements at the depth of the transducers. These values were directly read and incorporated into the SWATHplus acquisition software giving real-time speed of sound at the transducer while underway. In addition, a separate sound velocity profiler (Valeport miniSVP) was used to collect speed of sound profiles (water surface to seafloor) at intervals throughout the survey.

Swath Bathymetry Processing: Position data recorded by the Coda-Octopus F190R IMU system were corrected in real time via the OmniSTAR HP differential navigation system. The IMU also applied real-time motion corrections for heave, roll, and pitch to the vertical component of each position fix. The corrected positions were then integrated with the observed bathymetric values to calculate a final ellipsoid height and position representing the elevation of the seafloor with respect to the geodetic reference frame ITRF05 across the swath range. SWATHplus serves as both an acquisition software and initial processing software. Preliminary roll calibration trackline data were collected and processed using Systems Engineering and Assessment, Ltd. SWATHplus and Grid Processor software version 3.7.17. Instrument offset and calibrations values were input into the session file (.sxs), and the raw data files (.sxr) were then processed using the updated system configuration 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 or acoustic filtering applied in SWATHplus is also written to the processed data file (.sxp), which can then be imported into advanced sounding data processing software such as CARIS HIPS and SIPS. The initial real-time processing datum for the swath and backscatter data was ITRF05, which is the acquisition datum for OmniSTAR HP position and navigation data. All processed data files were imported into CARIS HIPS and SIPS version 7.1, and the original sounding data were edited for outliers using the program's depth filters and reference surfaces. Any remaining outliers were then edited out manually. A CARIS BASE (Bathymetry with Associated Statistical Error) surface with associated CUBE (Combined Uncertainty and Bathymetry Estimator) sample surface was created from the edited soundings dataset. A BASE 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 5-m resolution CUBE surface was created to perform initial hypothesis editing using the CARIS Subset Editor tool, followed by higher resolution surface detail editing within subset editor. The x,y,z data were exported as ASCII text at a 5 x 5 m sample resolution in the ellipsoid datum of ITRF05.

Differentially Corrected Navigation Processing: The coordinate values of the GPS base station (TMRK) are the time-weighted average of values obtained from the National Geodetic Survey On-Line Positioning User Service (OPUS). The base station coordinates were imported into GrafNav version 8.1 (Waypoint Product Group) and the kinematic GPS data from the survey vessel were post-processed to the concurrent GPS session data at the base stations. During processing, steps were taken to ensure that the trajectories between the base to the rover were clean, resulting in fixed positions. By analyzing the graphs, trajectory maps, and processing logs that GrafNav produces for each GPS session, GPS data from satellites flagged by the program as having poor health or satellite time segments that had cycle slips could be excluded, or the satellite elevation mask angle could be adjusted to improve the position solutions. The final differentially corrected precise DGPS positions were computed at 1-second (s) intervals for each rover GPS session and exported in ASCII text format to replace the uncorrected real-time rover positions recorded during acquisition. The GPS data were processed and exported in the World Geodetic System of 1984 (WGS84) (G1150) geodetic datum.

Single-beam Bathymetry Processing: The final DGPS positions exported from GrafNav were merged with the raw HYPACK files using the System for Accurate Nearshore Depth Surveying (SANDS), version 3.2. SANDS is a single beam acoustic (sounding) GPS-based hydrographic processing software developed by the USGS for shallow water bathymetric mapping (Hansen, 2008). Data were merged, geometrically corrected, and exported from SANDS in the originally acquired ellipsoid datum of WGS84 (G1150), which is equivalent to ITRF00.

Single-Beam Bathymetry Error Analysis: The processed single-beam data points were imported into ArcGIS, and an in-house Python script was created to evaluate the elevation differences at the intersection of crossing tracklines. The script calculates the elevation difference between points at each intersection using an inverse distance weighting equation. Elevation values at line crossings should not differ by more than the combined instrument acquisition error (manufacturer specified accuracies). GPS cycle slips, stormy weather conditions, and rough sea surface states may all contribute to poor data quality. If discrepancies were found that exceed the acceptable error threshold, then the line determined to be in error was either statically adjusted or removed. A line in error is considered to have one or more of the following: (1) a segment where several crossings are incomparable, (2) a known equipment problem, or (3) known bad GPS data seen in the post-processing steps.

Datum transformation: Using the transformation software VDatum version 3.2, both the interferometric swath and single-beam x,y,z data were transformed horizontally from their data acquisition datums (swath, International Terrestrial Reference Frame of 2005, ITRF05; single-beam, ITRF00) to the North American Datum of 1983 (NAD83) reference frame and the North American Vertical Datum of 1988 (NAVD88) orthometric elevation using the National Geodetic Survey (NGS) geoid model of 2009 (GEOID09).

Gridding Bathymetric data: Using ESRI ArcGIS version 10, the swath and the single-beam elevations were examined for spatial distribution and vertical agreement. After combining the datasets, a triangulated irregular network (TIN) was generated to provide unassuming linear predictions over data gaps in order to aid in the process of gridding the data into a raster layer or DEM. Converting the data points to a TIN surface also disallows any predictions outside the actual data extent. Using the natural neighbor algorithm in ArcGIS software, a DEM was generated at 50 x 50-m cell resolution. Finally, a low-pass raster-data filter was used to improve the uncertainty over data gaps between tracklines due to large trackline spacing relative to the swath width during acquisition, which is inherent to shallow water surveys over a large area.

Added keywords section with USGS persistent identifier as theme keyword.