Hillshaded relief produced from bathymetric data collected by the U.S. Geological Survey offshore of the Chandeleur Islands, LA, 2006-2007 (BATHY_HILLSH.ASC, ESRI ASCII GRID)

A suite of processing software called SwathEd (version: none) developed by the University of New Brunswick - Ocean Mapping Group (<www.omg.unb.ca/~jhc/SwathEd.html>) was used to process, edit, and archive the bathymetric soundings after collection (Beaudoin, 2002). The processing and editing steps were: 1. The raw Submetrix 2000 Series data were 'unraveled'. This process creates several files on disk: bathymetric soundings, raw sidescan-sonar backscatter, navigation, parameter files describing the configuration of the system (i.e. offsets between motion reference unit and GPS antenna, etc), and sound velocity at the transducer and sound velocity profiler information. 2. The navigation data were edited on screen using the SwathEd routine 'jview' to remove undesirable points, including turns at the ends of survey lines. Jview also rejects stray GPS fixes outside of the survey area - a parameter set by the operator. 3. The soundings for each trackline were edited using SwathEd. Blocks of data across and along a trackline can be displayed in a SwathEd window. Anomalous points were identified by comparison to other points (i.e. standard deviation of a swath section) and removed in a process known as 'fluffing'. 4. The bathymetric soundings from each processed data file were mapped onto a Mercator grid (UTM zone 16N) from the original acquired projection which was geographic WGS84 using weigh_grid with grid nodes spaced at 5 meters. The weigh_grid program creates a Digital Terrain Model (DTM) by summing up the weighted contributions of all the provided data into the DTM. The weighted contributions only extend to a user defined region around the true location of the estimated depth value. Processing took place in 2006 and 2007.

The data were corrected for errors in soundings due to sound refraction, caused by variations in sound velocity profile, using the SwathEd refraction tool (version: none). These artifacts can be recognized in a cross-swath profile of a relatively flat patch of sea floor. When viewing the swath data across a profile, the sea floor will appear to have a "frown" or "smile" when in fact the data should be flat across the profile. Insufficient and/or erroneous sound velocity information, which is usually due to too widely spaced or non-existent velocity profiles within an area, results in an under or over-estimate of water depth which increases with distance from the center of the swath. For a discussion of how this effect can be recognized in a swath bathymetric data file, see <http://www.omg.unb.ca/AAAS/UNB_Seafloor_Mapping.html>. 2. The bathymetric data were corrected to mean lower low water using discrete tidal-zoning models (DTZ) developed by NOAA/NOS's Hdrographic Planning Team (Martin, personal communication, 2006; 2007) (<http://www.tidesandcurrents.noaa.gov/hydro.html.). The DTZ models corrected differences in tidal phase and amplitude between the survey areas and observations made at NOS tide gauges at Gulfport Harbor (8745557) in 2006, and Bay Waveland Yacht Club (8747437) in 2007. The tidal zone models were imported into CARIS (version: HIPS and SIPS 6.1) as text files and used to correct tidal observations for each survey. The corrected values were then exported from CARIS as ASCII text files and converted to binary files (using SwathEd 'binTide') that could be imported into SwathEd. 'mergeTide' in SwathEd was used to apply the corrected tidal data to the swath soundings creating new sounding values relative to MLLW. 3. A 5-meter grid of tide corrected bathymetric soundings was created using weigh_grid. 4. The processed 5-m bathymetric grid was converted from OMG .r4 format to ASCII x,y,z data in DMagic (Data Magician (version 6.4.0a, Build 474 Professional)). Processing took place in 2006 and 2007.

The 5-m ASCII x,y,z file (for each cruise; 2006 and 2007) was gridded in GMT (version: 4.2.1; Wessel and Smith, 1998) using 'blockmedian' with a cell size of 50 meters. This routine was conducted to minimize edge effects in the 'surface' gridding routine because these data were collected at approximately 45-50% seafloor coverage. Next a tension spline routine in GMT was used ('surface'). The tension was set to 0.25.

The GMT grid (*.grd) was then converted to an ESRI Binary Grid via Mirone (version 1.2.3; Luis, 2007) and Matlab R2007b (version 7.5.0.342) - save grid as - ArcGIS binary grid. The projection was defined in ArcMap (version: 9.2; build 1420) using Data Management Tools - Projections and Transformations - Define Projection: UTM, zone 16N, WGS84 Datum. A hillshade was then created using Spatial Analyst: Surface Analysis - Hillshade with an azimuth of 350 and an altitude of 45. The cell size remained at 50 meters.

The ESRI Binary Grid was converted to an ASCII grid with an ESRI header using ArcToolbox (version 9.2; build 1420) -- Conversion Tools -- From Raster -- Raster to ASCII; in order to distribute these data via zipped files.

Edits to the metadata were made to fix any errors that MP v 2.9.34 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.). The link to the data in the Identification_Information section had to be fixed to link to the correct dataset. The format of the time period of content had to be fixed. Formatting of the cross-references was fixed to meet the metadata standard. The source information was incomplete and had to be modified to meet the standard. 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.