Interpolated swath bathymetry hillshaded image collected by the U.S. Geological Survey - Woods Hole Coastal and Marine Science Center surrounding the nearshore of the Elizabeth Islands, MA, 2010 (ei_2hm_fillhs.tif, GeoTIFF)

SwathPlus Software (version: 3.7.0) was used to acquire the raw swath data (*.sxr) and output a processed soundings file (*.sxp). Several bathy filters were applied to the processed file within SwathPlus to remove erroneous soundings and reduce the density of the data. Corrections were also applied for speed of sound changes (using sound velocity profiles), ship motion, GPS antennae offsets, and transducer depth and angle below the surface. Bathy filters included a low amplitude (100%), range (1-m below the surface), box (2-m to avoid nadir), alongtrack 1 (depth difference of 10-m, window size 10-m, and learn rate of 0.6), alongtrack 2 (depth difference of 1-m, window size 1-m, and learn rate of 0.9), and a mean filter (0.1-m).Several lines (l91f1-l100f1) were in very shallow water between Nashawena and Cuttyhunk. The filters used above for the majority of the survey were not tracking the seafloor well in very shallow water. All filters except the low amplitude and mean filter were turned off for these shallow water lines, to allow more data into the processed file. This procedure resulted in more editing later in CARIS, but it preserved more good data as well.

CARIS (version: HIPS and SIPS 7.0; service pack 2) was used to further process, finalize, and create a surface from the bathy files (*.sxp) from SwathPlus. CARIS processing with HIPS (hydrographic information processing system) has a workflow that ensures the user has done all necessary corrections to the soundings before creating a final gridded surface. The workflow starts with creating a vessel file (the R/V Rafael for this cruise). The vessel file contains any information related to GPS, MRU, and water level offsets that weren't included during acquisition using SwathPlus. Additionally, the user can report any errors in offsets (i.e. MRU to transducer measurement error, usually around 1-2 cm), which will later be used to calculate TPU (total propagated uncertainty), which is a requirement for CUBE (Combined Uncertainty and Bathymetry Estimator) editing (Calder, 2003). The next steps in the HIPS workflow include creating a project and converting the *.sxp data to CARIS HDCS (hydrographic cesar) format. Sound velocity corrections are the next step in data processing. Errors in soundings due to additional sound refraction not accounted for by sound velocity profiles were done using the CARIS refraction editor. 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 results in an under or over-estimate of water depth, which increases with distance from the center of the swath. Next a tide correction was applied in CARIS (described in the next process step) and merged with the soundings, and then the TPU was calculated in order to utilize the CUBE editor, which is an uncertainty based data cleaning tool that creates a 'best estimate' surface for the soundings. The cube surface is then used to further QC the soundings. Some lines required swath editing or subset editing as well as automated filtering, especially in the shallow water areas (l91f1-l100f1). The final step in the workflow was to finalize the surface (at 2.5 m resolution) and export it from CARIS in XYZ format, which can then be imported to other programs.

In order to apply a tidal correction to the soundings, an offset from Mean Sea Level MSL) to Mean Lower Low Water (MLLW) (-0.676 m) was entered into the HYPACK configuration file for the RTK device (dev 0) setup prior to the survey. This value was calculated by setting a DGPS antenna on a number of survey stations around Buzzard's Bay and using the bench mark sheets in conjunction with the DGPS height calculations to determine the offset from MLLW to MSL. Calculated HYPACK RTK water level heights were then extracted from the raw HYPACK files using an AWK script (parseNovatelHypackTides.awk), which calculates the MLLW tide value for each Julian day of the survey. An offset of 2.9 meters was added to each calculated tide value (TID 0) to compensate for the RTK antenna to water line offset, and then the sign was inverted. Since HYPACK reports water level heights relative to MLLW, heights above the MLLW datum are reported as negative values because they are above the datum per standard surveying XYZ convention. The file resulting from running the AWK script produces a file of MLLW tidal height corrections formatted in the CARIS TID file format: YYYY/MM/DD HH:MM 0.00 where 0.00 is the correct water level height. The RTK heights were then merged into the processed CARIS depth soundings in the bathy processing workflow using the Tide load and Merge functions.

The 2.5 meter tide corrected base surface that was finalized and exported from CARIS was gridded in ArcMap (version 9.3) using 3D Analyst - natural neighbors. An output cell-size of 2.5 meters was used, and NoData values were interpolated within the domain. A shapefile polygon representing the survey boundary was used to clip the grid to the extent of the data. Next, also using 3D Analyst, a hillshade was created using Surface Analysis - Hillshade. The azimuth was 315, the altitude was 45, and the z-factor was 2. The hillshade grid was converted to a TIFF by right-clicking on the grid in the table of contents and choosing "export data".

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.). Fixed a link to the project page in the abstract. 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. Minor fixes to the attribute format for some attributes were needed. 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.