establish a shoreline change rate. The second category of error is that inher- 

 ent in the technique used, e.g., tracing and measuring errors in the base maps. 



Ground survey data and aerial photography for Ludlam Beach, New Jersey, 

 analyzed by Everts, DeWall, and Czerniak (1980), provided a means to evaluate 

 the effectiveness of the use of the waterline and wetted bound obtained from 

 aerial photos to establish shoreline change rates as measured on the ground. 

 In addition, their photo analysis evaluated the procedures used in the base 

 map technique, and the results are as follows: 



(1) Over a long study period, the waterline and wetted bound 

 equally reflect average beach changes as determined by the MSL 

 shoreline survey results, where the field survey technique is the 

 most accurate. 



(2) The wetted bound showed less longshore variability in posi- 

 tion than the waterline (variability being measured by the mean 

 standard deviation) . The difference between the amount of variability 

 shown by the wetted bound and survey MSL shoreline was not statis- 

 tically significant. 



(3) No significant error due to reproduction using a Xerox 1860 

 Printer was found in an evaluation of the base maps. The mean 

 difference was 0.34 percent (i.e., 0.34 centimeter in 100 centimeters 

 on the map). Therefore, beach features from the aerial photo sets 

 were traced on nearly identical base maps, and measurements were 

 made at nearly the same points on each base map. 



(4) A possible cause of error in an analysis of beach features on 

 aerial photos is the inability to consistently select the position of 

 the feature at the exact location. The least selection difficulty 

 occurred in establishing the waterline position. The most difficult 

 to select was the location of the longshore bar. The mean error in 

 selecting the position of the beach features was 0.01 centimeter 

 (absolute) . No real difference was found in the measurements made 



in "complicated" areas and those made in "simple" areas (sharp, 

 shore-parallel, straight-line features) . 



(5) Measurement error, i.e., repeatability errors in measuring 

 distances on the base maps, varied from to 0.0015 centimeter 

 (absolute). The average error was 0.00023 centimeter. 



V. DISCUSSION 



The base map technique allows a visual comparison of features along the en- 

 tire study shoreline, i.e., beyond the bounds of the individual photos, as well 

 as a means to quantitatively measure changes in the features through time. This 

 freedom also allows lesser known relationships to be studied. For example, bars 

 and breaking waves show a natural alongshore continuity and, by analyzing the 

 base maps, it is possible to measure how the position of such features change 

 as a function of time and location. The base map analysis of the crest orien- 

 tation of incoming waves will provide information about wave refraction, a task 

 which might be difficult to do by analyzing the photos directly. 



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