INCREASING THE ACCURACY AND RESOLUTION OF COASTAL BATHYMETRIC SURVEYS

The development of coastal sediment budgets and models for sediment tr ansport and shoreline change require bathymetric surveys with vertical resolution and accuracy of 5 cm or better. Horizontal resolution and accuracy need to be at least 10 cm to quantify bedforms and bars. Sled s are probably the most accurate, widely used system for nearshore sur veys, but their contact with the bottom limits their speed, spatial re solution, and ability to operate in many situations. Boat-based echo s ounder surveys can achieve a higher spatial resolution and can operate where sleds cannot, but waves, tides, and other water-level fluctuati ons as well as boat dynamics and variations in the speed of sound in w ater can greatly limit their accuracy. Problems related to a survey sl ed's contact with the bottom cannot be overcome; therefore, echo sound er surveys must be improved. The newly developed high-accuracy, high-r esolution bathymetric surveying system (HARBSS) is designed to overcom e the confounding effects of changing vessel draft, waves, and tides o n depth soundings and to eliminate the need for measuring and modeling water level for a particular survey. The system combines Global Posit ioning System (GPS) receivers, an electronic motion sensor, a digital- gyro compass, a digital-analog echo sounder, a conductivity-temperatur e-depth probe (CTD), a computer, and custom software. The GPS antenna, compass, and motion sensor are aligned with the echo sounder's transd ucer. Using a bias-free phase solution from the GPS data (X,Y,Z accura cy of better than 1 cm), attitude information from the motion sensor, and heading information from the compass, the position and aim of the transducer is determined for each sounding. The CTD provides data to c alculate the speed of sound. Using the above data, the sounding depths and horizontal locations of sounding: points are corrected in XY, and Z with respect to an Earth-centered ellipsoid. In constant and unifor m speed-of-sound conditions, HARBSS can provide soundings that are wit hin 5.2 cm (mean error of 3.7 cm) of their true elevations. Horizontal accuracy is estimated to be within 10 cm. This accuracy can be achiev ed from a small, open boat that is rolling, pitching, heaving, or list ing. Error analysis indicates that we may be able to decrease the erro r by one half with better synchronization and interpolation of the var ious data streams.