1 meter resolution GeoTIFF image of the sidescan sonar backscatter imagery of Boston Harbor and Approaches (BH_1MBS.TIF, UTM 19, WGS84)

Sidescan data were acquired by NOAA NOS with the Edgetech model 272-T (100 kHz) or the Klein T-5500 (455 kHz) sidescan sonar systems. Equipment was either hull mounted or towed. Range, layback, gain and other acquisition parameters varied and were adjusted accordingly in the acquisition software. Raw .xtf sonar files were transfered from NOAA NOS to the USGS/CZM.

The raw sidescan-sonar data were processed using XSonar/ShowImage software. The processing steps included demultiplexing the raw sidescan data using a median filtering routine to suppress speckle noise, a file check of navigation points to delete repeated fixes or other logging errors, correct for slant-range distortion, signal attenuation, and dropped sonar lines (Danforth, W.W., 1997, Xsonar/ShowImage: A complete system for rapid sidescan-sonar processing and display: U.S. Geological Survey Open File Report 97-686, 77 p.).

PCI Geomatics is a commercially available software package used to digitally mosaic sidescan-sonar records. The geographically oriented raster imagery is imported as individual line files. Adjacent sidescan-sonar records are compared to establish ground control points at common target locations. This helps to correct for navigational errors and potential feature offset due to undefined layback of the tow vehicle. A stencil line is drawn around an individual swath record to crop and paste it on an adjacent swath. This procedure is repeated for each sidescan-sonar line swath to form a composite digital mosaic of the area.

A linear stretch was applied in to the sidescan sonar image to make adjacent areas mapped with different sonar systems internally consistent. The processed data from a given system, in many cases, were constrained to a limited range of backscatter intensity values. Adjustments were made to the greyscale imagery to tone-match and keep each survey internally consistent. The process described above for mosaicking in PCI was then repeated of each area mapped by a specific system and the patchwork of sidescan sonar images were combined into a composite image for the entire study area.

The final mosaic was brought into Adobe Photoshop where a linear stretch was applied to the data. The data values were manipulated to shift a larger range of pixel values within the 8-bit range that represent the data. This enhances the visual quality of the image and maximizes the contrast between areas that may originally have small tonal differences.

Edits to the metadata were made to fix any errors that MP v 2.9.30 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. In this particulary metadata file, none of the original process steps had a process date, so unknown was used for all those process steps. The source citation of the source contribution was blank, so that information was populated based on the information contained in the process steps. Other minor edits were probably performed (title, publisher, publication place, etc.). 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.