1-m Bathymetric Grid Generated from National Oceanic and Atmospheric Administration (NOAA) Survey H12007 in the Vicinity of Cross Rip Channel, Nantucket Sound (H12007_GEO, Geographic, WGS84)

Two 8.5-m aluminum Jensen launches and the NOAA Ship Thomas Jefferson were used to acquire the multibeam echosounder (MBES) bathymetric data for survey H12007 during April-May 2009 in central Nantucket Sound, offshore southeastern Massachusetts. The MBES data on launch 3101 were collected with a hull-mounted RESON SeaBat 455-kHz 8125 shallow-water system. This system measures two-way sound travel time across a 120-degree swath. The SeaBat 8125 has 240 beams with a cross-track beam width of 0.5 degrees at nadir. The MBES data were collected aboard the ship and launch 3102 with RESON 7125 systems which measured two-way sound travel time across a 140-degree swath. Original horizontal resolution of the MBES data was 0.5 m for depths less than 20 m and 1 m for the deeper areas. Vertical resolution of the MBES data is about 0.5% of the water depth. The bathymetric data were acquired and logged with HYPACK Hysweep and processed by NOAA using CARIS HIPS (Hydrographic Image Processing System) software for quality control, to incorporate sound velocity and tidal corrections, and to produce the continuous digital terrain model. All positioning and attitude were determined with Trimble DSM212L DGPS receivers and Applanix POS/MV Model 320 v.4 inertial navigation systems; HYPACK 2009 was used for acquisition line navigation. Sound velocity corrections were derived using frequent SEACAT CTD (conductivity-temperature-depth) profiles. Typically, a CTD cast was conducted every three to four hours of MBES acquisition. Tidal zone corrections were calculated from the tide station at Nantucket Island, Massachusetts. Vertical datum is mean lower low water. Individuals interested in detail descriptions of the MBES acquisition and processing should consult the descriptive reports. The data were gridded to 1-m resolution and saved as a CARIS HIPS/SIPS v6.1 SP2 database in UTM Zone 19, NAD83. Vertical offsets are present in the data, but none exceed the permissible error limits. The sources of these offsets include 1) change and acceleration of current flow in the narrow channel that changed the draft of the survey vessel, 2) shifting of sand waves due to strong currents, 3) a consistent 15-20 cm positive bias in the data from launch 3102, and 4) a faulty RESON 7125 multibeam receiver on the Thomas Jefferson.

The H12007 CARIS HIPS base surface H12007_1_CUBE_NOAA_1m_Final (fieldsheet H12007_Combined) was opened in CARIS HIPS/SIPS v7.0.2 and exported as a Bathymetry Attributed Grid (.bag) file using the CARIS HIPS Export Wizard.

The H12007 .bag file was then imported into the DMagic v7.2 program and converted to a Scientific Data Object (.sd) file.

The H12007 Scientific Data Object (.sd) surface was then exported as an ESRI ASCII raster file using the DMagic v7.2 program.

The ESRI ASCII raster file was then imported into ArcCatalog v9.3 and converted to an ESRI 32-bit floating point grid using the ASCII to Raster conversion tool (ArcToolbox - Conversion Tools - To Raster - ASCII to Raster).

Defined the projection as UTM Zone 19, NAD83 using ArcCatalog v9.3 (ArcToolbox - Data Management Tools - Projections and Transformations - Define Projection).

The UTM grid was reprojected into a geographic grid, WGS84 using ArcCatalog v9.3 using Data Management Tools, Projections and Transformations, Raster, Project Raster). The resampling technique was bilinear; the transformation was NAD_1983_To_WGS_1984_1 #; other used parameters were defaults.

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.). 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.