(A.) Demultiplex, or unravel, the acquired Simrad data files using RT to generate separate files containing navigation, depth soundings, sidescan sonar backscatter values, and sound velocity information.
(B.) Automatically reject bad data (autoRejectSoundings). For the multibeam soundings, reject data outside expected depth ranges (operator's decision based on nautical chart data); for navigation data, reject fixes with poor GPS statistics.
(C.) Edit the navigation data on-screen using jview to remove undesirable points, including turns at the ends of survey lines.
(D.) Edit the multibeam soundings on-screen using swathed to remove individual anomalous soundings.
(E). Merge tidal information and the corrected navigation back (mergetide and mergenav) into the data files. Tidal information was obtained from the NOAA tide server using tide station 8531680 located at Sandy Hook, //www.co-ops.nos.noaa.gov/) Final tidal corrections were made in the lab using a different procedure (see below).
(F.) Map the bathymetric soundings from each processed data file onto a Mercator grid using weigh_grid with node spacings and scale selected by the operator.
(G.) Map the extracted sidescan sonar backscatter values onto a digital mosaic using mos2 in the Mercator projection at a scale selected by the user.
(H.) Using addSUN, generate bathymetric raster files using the mapped grid node information to depict the depth information in a shaded relief Mercator map. A Mercator projection allows individual map areas to be joined edge to edge when creating a composite image. The shaded relief images were generated using a sun elevation angle of 45 degrees from an azimuth of 0 degrees, and a vertical exaggeration of four times to emphasize sea floor features.
(I.) Generate a false colored image using mix_ci by combining the bathymetric and backscatter raster mosaics into a single image, also in the Mercator projection.
(2) Data processing and analysis in the lab included:
(A.) Removing sound refraction artifacts from the data (using the refraction tool in swathed) due to insufficient sound velocity profile information and varying water masses within the study area.
(B.) The measured elevations were adjusted for fluctuations in sea level during the survey by subtracting tidal elevations predicted by a tidal model and low-frequency sea level observed at the National Oceanic and Atmospheric Administration Sandy Hook tide station located at 40 degrees 28 minutes N., 74 degrees 0.6 minutes W. The tidal model utilized 9 constituents derived from a 4-month bottom pressure record obtained at Station A, located at 40 degrees 23.4 minutes N., 73 degrees 47.1 minutes W. in 38 m water depth about 2.7 km east of the HARS, during the winter of 1999-2000. An estimate of the error due to sea level remaining in the multibeam observations after the sea level correction is about 3 cm.
(C.) Two corrections have been made to the HARS multibeam data as published in Butman and others (2002). (1) The backscatter intensity data between 180-220 was stretched to 0-255 (The data published in 2002 between 180-225 was stretched to 0-255). The shaded relief data between 150-200 was stretched to 0-255 (as in the 2002 data). Aging of the multibeam transducer and software changes implemented to correct the near-nadir response make the backscatter intensity observed in 1996, 1998, and 2000 not directly comparable. Further work is needed to match the backscatter intensities between the 1996, 1998 and 2000 surveys. (2) The data were also reprocessed to correct for an error in the UNB software that projected the data on a sphere rather than on the WGS84 ellipsoid. The horizontal error in placement of the data published by Butman and others (2002) was 0 in the northwestern corner of the HARS, and reached about 12 m in the southeastern corner.
All mapped files are in the Mercator projection, having a central longitude of -75 degrees West, a latitude of true scale of 40 degrees north and the horizontal datum is WGS84. The vertical datum is mean lower low water.