Survey lines along which Klein 3000 sidescan-sonar data were collected by the U.S. Geological Survey offshore of Massachusetts between Duxbury and Hull (DH_KleinTrackline shapefile, Geographic, WGS84)

Field activity 06012: The Klein 3000 was towed from the stern A-frame of the R/V Megan T. Miller. Position data were provided by the Z-Surveyor Extreme for real-time kinematic (RTK) navigation and recorded to the raw data files (XTF) via SonarPro (version 9.6) on the sonar acquisition computer. During some portions of the survey, technical difficulties caused the GPS systems to malfunction; in some instances the acquisition systems fell back on the RTK Float or the autonomous GPS navigation; but in some cases surveying was halted until a reliable GPS signal could be achieved. Layback positions, which account for the linear distance between the shipboard RTK-GPS receiver and the towed sonar vehicle were measured by a cable counter on the winch that was used to deploy the sidescan-sonar towfish. Layback offsets were directly applied into the acquisition software so the navigation recorded in the XTF data reflect the position including the layback offset. A conservative estimate of positional accuracy is estimated to be ± 10 m although when the system was recording RTK navigation the positional accuracy is likely <1 m as RTK-GPS provides horizontal positional accuracies on the decimeter scale. Positional uncertainty arises because the layback calculations do not account for fish motion behind the vessel, which is caused by sea state and vessel speed induced changes in the angle and scope of the tow cable. Field activity 07001: The Klein 3000 was towed from the port side aft on the R/V Rafael at a fixed distance that was accounted for during acquisition. Position data were provided by the Ashtech Extreme for real-time kinematic (RTK) navigation and recorded to the raw data files (XTF) via SonarPro (version 10.0) on the sonar acquisition computer. A conservative estimate of positional accuracy is estimated to be ± 10 m although when the system was recording RTK navigation the positional accuracy is likely <1 m as RTK-GPS provides horizontal positional accuracies on the decimeter scale.