St. Petersburg Coastal and Marine Science Center
System for Accurate Nearshore Depth Surveys (SANDS)
Advanced swath bathymetry methods are not efficient in shallow water because the swath width becomes increasingly narrow. A System for Accurate Nearshore Depth Surveys (SANDS) was developed at the USGS for operation in shallow, 0.5 - 10 m, water. A shallow-draft boat (Streeterville) is equipped with a geodetic Differential Global Positioning System (DGPS), a digital heave/roll/pitch sensor, and a digital fathometer. Single depth measurements are collected at 10 Hz, or approximately every 3 meters, along a track line. This method achieves a depth accuracy of 8 cm and a horizontal accuracy of 4 cm in shallow and nearshore areas. DGPS reference stations must be established within 10-15 km to achieve the highest vertical accuracy.
Seafloor grids are produced from the track lines. The shallow seafloor maps can be used to evaluate changes in depth and bottom characteristics and to understand the processes of sedimentation and movement of material over time.
Water clarity is not an issue with acoustic instruments. An important advantage of the acoustic mapping method is the ability to measure depth to bottom in turbid waters. See example at: http://sofia.usgs.gov/publications/ofr/00-347/