Hillshade of Swath Bathymetry collected by the USGS offshore of the Grand Strand, South Carolina, 1999-2003 (BATHY_HILLSH, grid)

Bathymetric data were corrected for variations in the speed of sound throughout the water column; sound velocity profiles were acquired throughout the survey area using an Applied MicroSystems SVPlus Sound Velocimeter. These data were downloaded and reformatted to the required format needed for use within University of New Brunswick's Ocean Mapping Group (OMG) SwathEd multibeam processing software. The raw Submetrix 2000 Series data were "unraveled" and reformatted into the OMG format. This process creates several files on disk: 1) bathymetric soundings 2) raw sidescan-sonar backscatter 3) navigation 4) parameter files describing the configuration of the system (i.e. offsets between motion reference unit and GPS antenna, etc). This process also incorporates the sound velocity profiles in order to properly trace the rays given the structure of the water column.

The OMG formatted bathymetric data were then graphically edited using the SwathEd multibeam processing software. This enabled editing on a "ping-by-ping" basis, both in the across- and along-track dimension. Spurious or erroneous data were removed. The navigation data were also graphically examined for each line of data.

Navigation data output during the unravel were reformatted for entry into MATLAB. The time of day and position were used to extract the tidal elevation from within the ADCIRC circulation model. The ADCIRC circulation model is referenced to Mean Sea Level. Verified tidal data from the NOAA tidal gauge at SpringMaid Pier, SC were downloaded, referenced to Mean Lower Low Water (MLLW). A low-pass filter was applied to the NOAA tide data; the residual was assumed to represent the wind component of sea surface elevations. Tidal elevation (output from ADCIRC) and wind component (NOAA tide station) represent total variation in surface elevations. These data were then written to an ASCII file and reformatted to OMG format for use within SwathEd. The tidal data were then written into the headers of the OMG formatted files, using the 'mergedTide' command. (SwathEd Commands: binTide -year XXX adcirc_wind.txt tide.bin mergeTide -tide tide.bin *.merged ) SwathEd Software was used to grid the bathymetric data at 20 m grid cell size utilizing the 'weigh_grid' command. weigh_grid -omg -butter -power 2 -cutoff 10 -lambda 2.5 Grid *.merged The gridded data were then viewed using the SwathEd Jview Command. The PatchTool was used to assess the roll misalignment of the data; slight roll misalignment artifacts were visible in the gridded data. The PatchTool is a QC tool to assess the degree to which the head (transducers and MRU) needs to be 'rolled' to achieve the proper alignment. QC was also preformed on the navigation. Edits were made as necessary. After the needed adjustments were recorded, the configuration and navigation were remerged with the OMG bathymetric data using the mergeNav and mergeAtt commands. mergeNav -use_params 0 -nav *.nav -swath *.merged) The remerged data were regridded for a second round of QC. The roll misalignment artifacts were no longer visible in the gridded data. The bathymetric data to this point have been edited, navigation, edited, tidal elevations relative to MLLW incorporated in the data, sound velocity corrections applied, and roll misalignment rectified. Next the data need to be interpolated to generate a continuous bathymetric surface. The SwathEd 'make_blank' command was used to generate a 20 meter grid for the survey area. The OMG formatted bathymetric files were then written into the 20 meter grid using the 'weigh_grid' command. Due to noise within the far range of the sonar signal, the 'max_incidence' parameter was used. This essentially limits the across track data range of the data; thus elimination noise. (weigh_grid -omg -butter -power 2 -cutoff 20 -lambda 10 -max_incidence 60 Grid *.merged) The 20 meter gridded data were then exported to an xyz ASCII file. (r4toASCII -utm -merid -81 20m_grd 20m.xyz)

The 20m xyz ASCII data were then imported into Geographic Resources Analysis Support System (GRASS). A spline with tension and directionality interpolation routine was used in order to generate a continuous bathymetric surface from the interferometric sonar data. After many iterations, the gridding parameters resulting in a gridded dataset with the least amount of visible artifacts were as follows: s.surf.rst input=20m.xyz elev=grid100m field=1 tension=200 dmin=20 theta=45 scalex=0.5 smooth=2 A smoothing filter (comparable to a median/mean filter) was applied to the dataset.

The final grid was then exported as an ESRI Binary Grid. r.out.arc in=grid100m out=grid100m.asc The exported data were then imported to ArcGIS 9.0 using the ASCIItoGRID command. The projection was then defined: UTM, zone 17 projection, WGS84 Ellipsoid.

Hillshade was applied to bathymetric grid using an elevation of 45, vertical exaggeration of 5x, and sun angle of 45 within Spatial Analysis Tools within ArcGIS 9.1.

Edits to the metadata were made to fix any errors that MP v 2.9.32 flagged. This is necessary to enable the metadata to be successfully harvested for various data catalogs. In some cases, this meant adding text "Information unavailable" or "Information unavailable from original metadata" for those required fields that were left blank. Other minor edits were probably performed (title, publisher, publication place, etc.). An online link was changed to point to pubs.usgs.gov. Date range format had to be fixed. The distribution format name was modified in an attempt to be more consistent with other metadata files of the same data format. The metadata date (but not the metadata creator) was edited to reflect the date of these changes. The metadata available from a harvester may supersede metadata bundled within a download file. Compare the metadata dates to determine which metadata file is most recent.

Added keywords section with USGS persistent identifier as theme keyword.