HIGH-ACCURACY BATHYMETRIC SURVEYS FOR RESEARCH
Detailed comparisons of repeated bathymetric surveys are commonly inconclusive because the magnitudes of potential errors are equal to or greater than the actual changes of the seafloor morphology. The development of coastal sediment budgets and models for sediment transport 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. Sleds are probably the most accurate, widely used system for nearshore surveys, but their contact with the bottom limits their speed, spatial resolution, and ability to operate in many situations. Boat-based echo sounder surveys can achieve a higher spatial resolution and can operate where sleds cannot, but waves, tides, and other water-level fluctuations as well as boat dynamics and variations in the speed of sound in water can greatly limit their accuracy. Problems related to a survey sled's contact with the bottom cannot be overcome; therefore, echo sounder surveys must be improved.
We conducted a crossover analysis of the multiple transect data to determine the system's repeatability ( see above table). For this analysis, we avoided the spikes in the data by only considering data less than 170 m from the datum stake (see fig. 4). We extracted pairs of points obtained during separate passes along the transect that were closer than 10 cm, horizontally. Seventy-two pairs of points had a mean vertical difference of 5.2 cm, and the standard deviation of the vertical difference was 3.7 cm. If we include points within 50 and 100 cm of each other, the vertical standard deviation increases indicating horizontal repeatability of better than 10 cm.
To estimate accuracy, we conducted a crossover analysis between the multiple transects and the conventional ETS survey. Subtracting the heights of the ETS crossover points from the HARBSS points yields a mean height error of -1.2 cm. This is a measure of the bias of the HARBSS survey relative to the ETS survey. The bias is probably caused by either a tilting of the ETS survey rod or a small error (0.4 %) in the speed of sound setting of the echo sounder. The vertical difference between the HARBSS and ETS crossover points have a mean of 4.9 cm and a standard deviation of 3.7 cm. If we assume that the ETS survey represents the 'true' profile, the accuracy obtained during this survey is the same as our precision (repeatability) determined above.
We conclude that HARBSS can provide soundings that are within 5.2 cm (mean error) of their true elevations. Horizontal accuracy is estimated to be within 10 cm. This accuracy can be achieved from a small, open boat that is rolling, pitching, heaving, or listing. Error analysis indicates that we may be able to decrease the error by one half with better synchronization and interpolation of the various data streams and better incorporation of speed of sound data.
For more information, please contact Jeff Paine