Multibeam Bathymetry Data Collected in 2016 from Grand Bay Alabama/Mississippi: Adjusted processed elevation point data (x,y,z)

Acquisition: Multibeam data were acquired using a Teledyne Reson SeaBat T50-P multibeam echosounder mounted to the bow of the R/V Jabba Jaw, a 22-ft fiberglass shallow draft center console catamaran. An Applanix Pos MV Wavemaster II V5 Inertial Measurement Unit was mounted on top of the sonar bracket in the water and was used for positioning and attitude. Two Trimble 540AP GPS antennas were affixed to the top of the sonar mount pole. A functioning sound velocity sensor was not available for speed of sound readings at the head. A Sontek Castaway CTD was used for sound velocity profiles and speed of sound values. Hydrographic survey acquisition and processing software, HYPACK/HYSWEEP (version 2015) managed data inputs from sensors and produced two files (.RAW and .HSX). Data were collected in WGS84(G1674).

Processing: Positioning and attitude data were post-processed in Applanix POSPac MMS software version 8.0. Raw position files were imported into POSPac and processed with the Single-base function utilizing Continuously Operating Reference Station (CORS) ALDI (https://www.ngs.noaa.gov/CORS/Sites/aldi.html). ALDI coordinates were verified by use of the National Geodetic Survey (NGS) data sheets, the data were then processed using the Global Navigation Satellite System Inertial Processor function. Upon post-processing, a smoothed best estimate of trajectory (SBET) file was created and exported in WGS84 (G1674). Bathymetric data were post-processed using two separate hydrographic processing software systems. First, .HSX files were imported into HYPACK MBMAX64, version 2015, where sounding (x,y,z) position and motion data were corrected using the SBET file described above. Data were then cleaned using filters and manual editing. A sounding artifact near nadir was identified and manually removed. Data were then exported as a .HS2X file and imported into CARIS Hydrographic processing software version 10.2. Collected sound velocity profiles were applied to account for changes in the speed of sound within the water column during the survey. Refraction artifacts were present in the data and were resolved using the Refraction Editor. A roll calibration was performed and data were examined and further cleaned using Subset Editor. A 0.50-m bathymetric CUBE surface was computed and the final surface was exported as an x,y,z ASCII file using the Base Surface to ASCII function.

Static Adjustment: Post processed x,y,z data were imported into Esri ArcGIS version 10.3.1 where a point shapefile was created. A 30-m digital elevation model (DEM) derived from single-beam bathymetry data acquired in 2015 (DeWitt and others, 2016) was imported into the project and was super-sampled to a 1-m point data density. By use of a spatial join, all multibeam bathymetry data points which were within 1 meter from a single-beam bathymetry data point were extracted from the shapefile and saved for use in comparison analysis. This process was repeated for the single-beam points. The two derived shapefiles were joined, and the elevation fields were subtracted to obtain the static offset necessary to correct the multibeam soundings. These static adjustments (Site 1 = -0.411, Site 2 = -0.448, and Site 3 = -0.357) were then applied using the field calculator within the attribute table of the original multibeam file. Once corrected, the final x,y,z file was exported to an x,y,z ASCII format text file and prepared for dissemination.

Datum Transformation: Final x,y,z data were converted using the National Oceanic and Atmospheric Association (NOAA) VDatum software conversion tool version 3.7 (reported vertical transformation error is 5.4 cm) from the World Geodetic System of 1984 (WGS84 [G1674]) realization to the North American Datum of 1983 (NAD83[2011]). In addition, the data was transformed from WGS84 to NAD83 horizontal, and the North American Vertical Datum of 1988 (NAVD88), using the GEOID12A model (orthometric height).

For version 2.0 of this data release, the data download file name was changed from Grand_Bay_2016_MBB_xyz.zip to Grand_Bay_2016_MBB_Adjusted_xyz.zip. Files included within that zipped folder had "_Adjusted" appended to the original file names, which were: Grand_Bay_2016_MBB_WGS84_0.5m_xyz.txt, Grand_Bay_2016_MBB_NAD83_0.5m_xyz.txt, Grand_Bay_2016_MBB_NAD83_NAVD88_GEOID12A_0.5m_xyz.txt.

Added keywords section with USGS persistent identifier as theme keyword.