Single Beam Bathymetry XYZ Data Collected in June 2015 from the Chandeleur Islands, Louisiana

Three Geographic Positioning Systems (GPS) base stations were occupied during this survey (http://pubs.usgs.gov/ds/01039/html/ds1039_ overview.html). Benchmark MRK3 was located in the northern part of the survey area on the sound side of the islands near the Pelican fishing camp, SM01 was located in the middle of the survey area in the middle of the island at Monkey Bayou, and GRIG was located in the southern end of the survey on an abandoned oil drilling platform. The base stations were equipped with Ashtech Proflex GPS Receivers recording 12-channel full-carrier-phase positioning signals (L1/L2) from satellites via Thales Choke-ring antennas, recording at a rate of 0.1 seconds (s).

Single Beam Bathymetry Acquisition: The single beam bathymetric data were collected under the USGS FAN 2015-317-FA, which encompasses three sub-cruise identifiers specific to platform (15BIM01, 15BIM02, 15BIM03). Data were collected aboard three separate survey platforms: the 26-foot (ft) R/V Sallenger (cruise identifier 15BIM01), the the 17ft R/V Jabba Jaw (15BIM02), and the Personal Water Craft (PWC) R/V Chum (15BIM03), for a total of 1,614 trackline km. In total, the survey is approximately 2,400 km2, extending 3 km from the back-barrier shoreline of the islands and 5 km from the Gulf of Mexico shoreface. Shore-perpendicular survey lines were spaced approximately 500 m apart and shore-parallel crossing lines were located as close to the shoreline as possible, 1 km from the shoreline, and 3 km offshore of the Gulf shoreline. All data were collected and processed in the World Geodetic System 1984 (WGS1984 G1150). Boat motion was recorded at 50-millisecond (ms) intervals using a Teledyne TSS Dynamic Motion Sensor (TSS DMS-05) only on vessels 15BIM01 and 15BIM02. A motion sensor devise was not used for the PWC (15BIM03). Rather, to minimize motion errors, R/V Chum recorded GPS at a rapid rate of 0.1 s, utilized a short antenna height by use of a lever arm, and was outfitted with a narrow (4 degree) transducer beam. HYPACK version 14.0.9.47 (15BIM01, 15BIM02) version 13.0.09.17 (15BIM03), 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 an Odom echotrac CV100 sounder with a 200-kilohertz (kHz) transducer. Data from the GPS receiver, motion sensor, and fathometer were recorded in real-time aboard all vessels independently 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 profile (SVP) measurements were collected using two SonTek CastAway Conductivity, Temperature, and Depth (CTD) instruments as well as a Valport mini sound velocity profiler (SVP). The instruments were periodically cast overboard to observe changes in speed of sound (SOS). A total of 188 successful sound velocity casts were taken throughout the survey. Casts were taken at an average depth of 4.01 m, ranging from 0.13 m to 13.96 m and recorded an average sound velocity of 1527.93 meters per second (m/s) ranging from 1519.85 to 1539.23 m/s. The recorded profiles were later incorporated into the post-processing software to correct the depth for temporal and spatial SOS changes throughout the water column.

Differentially Corrected Navigation Processing: The coordinate values of the GPS base stations (MRK3, SM01, GRIG) are the time-weighted average of values obtained from OPUS. The base station coordinates were imported into GrafNav version 8.5 (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 and 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 were excluded, or the satellite elevation mask angle was adjusted to improve the position solutions. The final differentially corrected, precise DGPS positions were computed at the respective time intervals of the roving GPS (1.0 s for 15BIM01 (JD 168-171), 0.2 s for 15BIM01 (JD 172-175) and 15BIM02, and 0.1 s for 15BIM03) and exported in ASCII text format to replace the uncorrected rover positions recorded during acquisition. The GPS data were processed and exported in the World Geodetic System of 1984 (WGS84 G1150) geodetic datum.

Single Bream Processing: The raw HYPACK® data files were imported into CARIS HIPS and SIPS® (Hydrographic Information Processing System and Sonar Information Processing System) version 9.0.17. The corrected DGPS positions exported from GrafNav were imported into CARIS using the generic data parser tool. After parsing, the navigation data were scanned using the Navigation Editor, which allows the user to view multiple types of plots including trackline orientation, timing, and course direction. This check verifies if the parsed data corresponds to the processed DGPS. Next, Speed of Sound (SOS) Profile (SVP) casts were entered, and edited, using the SVP editor tool, and then applied as nearest in distance within time. All soundings are referenced to the ellipsoid (WGS84, ITRF00) during processing, this involved a step in CARIS to compute the GPS tide. The GPS tide represents the ellipsoidal surface. GPS tide and GPS height are then compared against each other to ensure correct computation by the program and applied GPS antenna height provided in the vessel file. All bathymetric data components; position, motion, depth, GPS tide, and Speed of Sound (SOS), were then merged and geometrically corrected in CARIS to produce processed x,y,z data. Once merged the dataset is reviewed for erroneous points using the Single Beam Editor. The points that are visually obvious outliers are often related to cavitation in the water column obscuring the fathometer signal, tight turns in the surf zone affecting the tracking of the incoming GPS signal, and/or false readings due to general equipment issues. Data showing these are either discarded or adjusted to surrounding sounding depths. Also, data points in areas of extremely shallow water (0.30 m to 0.50 m) such as shoals and seagrass beds are reviewed against the surrounding data for overall consistency. Finally, a Bathymetry with Associated Statistical Error (BASE) surface is created. Using the Subset Editor, the BASE surface is used as a color coded guide to pinpoint crossings that are visually offset from one another. If an offset is identified, it is further examined and is reprocessed if necessary. The geometrically-corrected point data are then exported as an x,y,z ASCII text file referenced to WGS84 (G1150), equivalent to ITRF00, UTM 16, and ellipsoid height in meters. The single beam bathymetry datasets combined (15BIM01, 15BIM02, and 15BIM03) consists of 8,223,749 point elevations with an ellipsoidal elevation range of -43.927 to -25.097 m.

Quality Control and Quality Assurance (QA/QC) and Datum transformation: All single beam data found in the ASCII exported from CARIS were imported into Esri ArcMap version 10.2, where a shapefile of the individual data points (x,y,z) was created and plotted in 1-m color coded intervals. First, all data were visually scanned for any obvious outliers or problems. A Python script was used for the purpose of evaluating elevation differences at the intersection of crossing tracklines by calculating the elevation difference between points at each intersection using an inverse distance weighting equation. GPS cycle slips, stormy weather conditions, and rough sea surface states can contribute to poor data quality. If discrepancies that exceed the acceptable error threshold were found, then the line in error was either removed or statically adjusted by the average of the crossings within the line. The script was run on all vessel point data first on a vessel by vessel basis and then run a final time for all data points from all vessels collectively. For this dataset, the collective run, using a merged shapefile of all vessel point data, is the most important as the survey structure did not allow for sufficient vessel crossing, rather vessels crossing other vessels. Once the dataset passed all QA/QC procedures and manual editing steps, the data were considered final.

Datum Transformation: The text files,15BIM01_SBB_Level_03_1039_ITRF00.txt,15BIM02_SBB_Level_03_1039_ITRF00.txt,15BIM03_SBB_Level_03_1039_ITRF00.txt, were converted using the National Oceanic and Atmospheric Association (NOAA) VDatum software conversion tool version 3.2 (reported vertical transformation error is 5.4 cm) from the International Terrestrial Reference Frame of 2000 (ITRF00) 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 2012 (GEOID12A).

Added keywords section with USGS persistent identifier as theme keyword.