The "hybrid" method indicates an analog fathometer record was made and 

 the survey vessel positioned by use of electronic ranging equipment. With this 

 method, a man onshore directed the vessel operator along profile lines, but the 

 vessel position was monitored by telemetering data to a field office where a 

 real time plot of position was produced by a minicomputer. Monitoring the sur- 

 vey in progress determined and eliminated both errors in positioning. An elec- 

 tronic timer at 2-second intervals simultaneously marked the analog record and 

 recorded the vessel's position on magnetic tape. 



The "automated" method indicates that the soundings from the fathometer 

 were digitized electronically and transmitted with position information to the 

 field office minicomputer. All survey data soundings as well as position were 

 monitored in real time. 



A LARC 5 amphibious vehicle was used in all three methods for the bathy- 

 metric part of the survey. The LARC allows the continued measurement of the 

 survey line through the surf zone, using rod and level methods when the 

 vehicle's wheels contact the bottom. Profile lines over the dry beach areas 

 were measured by standard level and rod transects. 



Late in phase I of the study, it was found the standard method was not 

 providing the reliable, accurate data desired. As shown in the plots in Figure 

 7, the data are on a line sheltered by the breakwater and at a point beyond 

 normal sand deposition. Divers measured the bathymetry using underwater ref- 

 erence stakes to verify no deposition at this point. These plots indicate that 

 unacceptable errors were introduced on several surveys. These errors were a 

 result of poor positioning data, poor fathometer calibrations, and errors in 

 data reduction. 



The standard surveying method assumes that the LARC is on line for all fixes 

 and that the vessel's position and the fathometer record have been marked both 

 precisely and simultaneously. However, this is not always true and errors that 

 occur can never be recovered. Even with no errors produced in field collection, 

 reduction of working scale plane-table data sheets was limited to an accuracy of 

 about 3 meters which was seldom attained. 



In the hybrid method, an electronic timer simultaneously marked the fathom- 

 eter at 2-second intervals and controlled the recording of the LARC's position, 

 eliminating human errors in determining position. Position accuracy, which was 

 limited by the electronic ranging equipment, was about 2 meters and consist- 

 ently attained. 



During the surveys, errors were also produced in fathometer calibrations 

 and data reduction procedures. Under the standard method, leadline soundings 

 were used for the first year (April 1974 to March 1975) for fathometer calibra- 

 tion. In taking leadline soundings, accuracy is determined by numerous factors, 

 including skill and care of the operator, m.otion of the vessel, and roughness 

 of the water's surface. Later surveys used a bar lowered at 5-foot intervals 

 under the fathometer for calibration. This type of calibration (bar check) 

 eliminates human factors and is complete in that it shows any occurrence of 

 nonlinearity in the fathometer. 



Uncertainty in interpreting the fathometer records is another source of 

 error which was discovered. Figure 8 shows fathometer records of two profiles, 



20 



