These data are high-resolution bathymetric soundings of the seafloor offshore of Massachusetts, from Nahant to Gloucester. Approximately 127 km² of the inner shelf were mapped in the nearshore region between the 10m and 40-m isobath.

These data were collected under a cooperative agreement with the Massachusetts Office of Coastal Zone Management (CZM) and the U.S. Geological Survey, Coastal and Marine Geology Program, Woods Hole Science Center. High resolution bathymetry, backscatter intensity, and seismic profile data were collected over 127 km² area covering a portion of the South Essex Ocean Sanctuary to characterize the surface and subsurface of the seafloor for regional geologic framework investigations. The purpose of this grid of hillshaded bathymetry is to provide a visualization of the vertical relief of the seafloor in the survey area. Bathymetric data were collected during two different surveys. Survey RAFA03007 began Monday, 22 Sep 2003 (JD267) and ended on Tuesday, 14 Oct 2003 (JD287). RAFA04002 began 17 Apr 2004 (JD 108) and ended on 2 May 2004 (JD123).

Data are not intended for navigational use. These data should not be used at resolutions for which it is not intended. Public domain data from the U.S. Government are freely redistributable with proper metadata and source attribution. Please recognize the U.S. Geological Survey (USGS) as the source of this information.

Vertical positional accuracy report:

Ship motion was recorded with a TSS DMS 2-05 Attitude Sensor, mounted on a head above the interferometric Submetrix 2000 Series transducers. Navigation was recorded using Real Time Kinematic Differential Global Positioning System (RTK-DGPS). The interferometric sonar is an angle-measuring system; depth accuracy decreases with increasing horizontal range. The combined angular accuracy of the Submetrix 2000 Series and the TSS DMS 2-05 attitude sensor is documented as 0.1 degrees. Assuming a constant angular accuracy, and using the International Hydrographic Standard (IHO) of a required 0.3 meter accuracy in < 30 meter water depth, all data collected within the survey fall within the IHO accuracy standards. However, vertical accuracy is also directly affected by the accuracy of both the navigation system and tidal measurements. RTK DGPS vertical accuracy is assumed to be 0.2 meters. Tidal elevations were derived from RTK -DGPS heights sent from a survey-specific base station at the Eastern Point Light House in Gloucester.

The RTK corrected signal was sent from a USGS survey-specific base station at the lighthouse on Eastern Point Lighthouse. The base station occupied a DMA monument, however, the x, & y positions were calculated for that location using a 3 day average of 12 hour data sets using the Online Positioning User Service (OPUS, National Geodetic Survey) solution. Tidal elevations were calculated from NOAA Tidal Station #8441841 using the same OPUS solution and then applied to the corrected elevations for the base station at Eastern Point Lighthouse. Antenna heights were referenced to Mean Tidal Level (MTL), and later reduced to Mean Lower Low Water (MLLW) for easier comparison of NOAA nautical charts.

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 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.

Soundings data were gridded using the weigh_grid routines from Swathed. Soundings were corrected to mean lower low water during the weigh_grid operation using the real time kinematic differential Global positioning navigation files. The final weigh_grid parameters used were: weigh_grid -omg -coeffs -butter -lambda 5 -cutoff 20 Max_incidence angle was not used in most lines to use all of the swath. Only lines from rough days or big heave were gridded with a small incidence angle to maximize the good data from adjacent lanes and reduce the noisy data.

Although this data set has been used by the U.S. Geological Survey (USGS), no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and/or related materials. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

se_5mbathhs-Nahant to Gloucester, Hillshaded bathymetry of the South Essex Survey Area
	Data format: Raster Dataset - ESRI GRID File or table name: se_5mbathhs Coordinate system: Universal Transverse Mercator Theme keywords: U.S. Geological Survey, USGS, Coastal and Marine Geology Program, CMGP, Woods Hole Science Center, WHSC, Bathymetry, Seafloor, Marine Geology, Swath Bathymetry, Interferometric Sonar, ArcGIS Raster, field activity serial number RAFA03007, field activity serial number RAFA04002, oceans and estuaries, elevation, ImageryBaseMapsEarthCover
Abstract: These data are high-resolution bathymetric soundings of the seafloor offshore of Massachusetts, from Nahant to Gloucester. Approximately 127 km² of the inner shelf were mapped in the nearshore region between the 10m and 40-m isobath.

FGDC and ESRI Metadata: