shimmer and the temperature gradients near the land-sea interface may also affect the 

 accuracy of the angular measurements. The authors have no quantitative feel for these sorts 

 of errors other than they are usually negligible relative to the other types of errors. These 

 errors may explain some systematic offshore changes amounting to several centimeters during 

 the summer months that do not appear to be related to the dominant processes. 



An additional, but unquantified source of error resulted from the thermal expansion and 

 contraction of the CRAB frame and its liquid-tilled tires. Although the CRAB's height was 

 routinely measured on land, there was no way to measure its height fully submerged. No 

 adjustment to the data has been made to account for this effect. The variation is suspected to 

 be on the order of 3 to 6 cm (0. 1 to 0.2 ft) between summer and winter extremes, but it 

 occurs gradually. This amount of variation, combined with the slight uncertainty of the over 

 300 different instrument setups, results in a survey noise level that obscures small bed-level 

 changes along the offshore reaches of the profiles. 



Error Identification and Correction 



Examples of typical nearshore survey errors and their impact are shown in Figure 7. 

 Errors in the data were most easily recognized through comparison with past data collected 

 for that profile. All data were compared to at least the previous survey of the profile. 

 Usually the data were error free. If not, this comparison showed where possible errors 

 occurred. The suspect points were then inspected more carefully. A decision was made as to 

 whether the point or points were in error or represented real changes. Figure 7 shows the 

 utility of comparison. Questionable data, where no clear error could be discerned, were 

 noted and compared to the next survey as well. This provided a double-check. Errors were 

 also identified using the measured location of the reference prism checked during the 

 instrument setup and throughout the survey. 



Corrections to the data consist of two primary types: deletion of points and subtraction or 

 addition of biases. Data points that were obviously erroneous were removed from the data. 

 These erroneous data points usually resulted from incorrect targeting of the Zeiss to the prism 

 cluster, improper adjustment of the Geodimeter tracker amplitude, or the use of points diat 

 were unaccountably, but obviously, wrong. Biases were either constant or gradual (distance 

 dependent) due to improper leveling of the instrument. Vertical errors due to leveling became 

 increasingly evident with increasing distance. 



Most often, constant vertical offsets were the result of either improper stationing of the 

 instrument (setting its elevation incorrectly), or rarely, improper entry of the elevation of the 

 prism cluster on the CRAB. These errors could be traced back to the data through the 

 recorded setup procedure. If there was no evidence of a mistake in stationing the instruments 

 or entry of the prism height, removal of the suspected bias was dependent on three factors. 

 The bias had to extend over the entire profile (past the normal closure point); the bias was 

 restricted to only one or two of the four profile lines; and there had to be no reason to expect 

 evidence of profile activity at depth. For instance, if the measured profile showed significant 

 erosion at a depth below the extreme profile closure depth during a period of below normal 

 wave activity, errors were strongly suspected. However, the bias was removed from the data 

 only after a second survey of the profile confirmed that an error had been made. 



Chapter 2 Profile Data "^ '' 



