A4 
deleting points that fell outside the overall envelope or had a “step” shape to 
them when compared to subsequent surveys of the same line. 
While these Geodimeter errors are unfortunate, their impact on the minigrid 
survey data is seldom > +15 cm in the vertical and is typically <+10 cm. 
Because of the height of the prism, data affected by the tilt of the CRAB on 
steep parts of the profile (particularly the foreshore and the bar slopes) were 
adjusted using an iterative technique in the analysis. The measured slope was 
used as a first guess to adjust the data points both horizontally and vertically. 
A new slope was then determined and a second pass, if required, was made. 
On a 10° slope typical of the beachface, the tilt can result in a +20-cm vertical 
adjustment to the data. 
13-m-Deep Survey 
The first 13-m-deep survey was conducted on 1 May 1990, followed by 
subsequent surveys on 2, 8, and 27 August 1990. All of the surveys were 
conducted with the Geodimeter 140-T, which was located on the end of the 
FRF pier and which tracked the sled as it was towed behind the LARC. 
The final 13-m deep bathymetry data were acquired on six different dates 
and include sled data from the three dates given above and CRAB survey data 
from the September FRF 26-line bathymetry survey and a preliminary minigrid 
survey on 19 September 1990. Unfortunately, the CRAB and sled data do not 
overlap well, resulting in discontinuities where the data sets meet. The 
difference is usually less than 20 cm, but the effect is quite pronounced on the 
bathymetric chart and on specific cross sections (an explanation for the 
discontinuities is given below). The most serious discontinuity, and the largest 
data gap, is around the 8-m pressure gauge array location. Neither the CRAB 
nor the sled were normally driven through the 8-m array. A special deepwater 
parallel pass was made alongside the array with the CRAB, but not until 
January 1991. In order to fill the data gaps in a reasonable way, a very large 
"search" area had to be defined in the gridding routine which was used to plot 
the data. This has the adverse effect of overly smoothing all of the data, 
particularly the irregular bottom found in the 10-to 13-m depth south of the 
pier. 
Problems with the sled were also encountered. These resulted primarily 
from the sled tilting sideways while on a turn, or backward when under tow. 
During the surveys on 2 and 8 August, strong currents caused the sled to tilt 
sideways when it was under tow. Consequently, these surveys were not used 
in the final data set unless they provided the only coverage of an area. Sled tilt 
results in a vertical error, which can show up as either a bump or a hole. If 
the tilt occurred gradually, it was difficult to identify and correct. 
Because of these problems, the data processing included rechecking and 
Appendix A Surveying 
