After all the bluff lines were drawn, a grid of the reference line and 

 station locations was placed on the November 1974 overlay and measurements to 

 each bluff line from each station were made. This procedure also eliminated 

 problems in relocating the reference line from photo set to photo set. 



2. Sources of Error . 



The straightforward estimation of distance from aerial photos is based 

 on several convenient assumptions which are not often satisfied practically. 

 Errors can be separated into those which are inherent in the photo and those 

 which are due to the analysis procedure. The various sources of error are 

 well known and frequently discussed (Thompson, 1966; Stafford, 1971; Istvan, 

 1974; Stoker, 1976; Tanner, 1978). Compensation for some errors may be possi- 

 ble if enough information is available. An estimate of the magnitude of the 

 maximum and most probable error for the procedure used is developed below. 



Photo errors are due to relief displacement and scale variations within 

 single photos, between photos in a set, and between sets. Since other errors 

 are scale-dependent, their effects have to be combined in any procedure where 

 measurements are made directly on the photos (Istvan, 1974) . 



By using the above procedure, variations in scale among photos in a set or 

 between sets were virtually eliminated (relative to other errors) by optically 

 matching the photos to the November 1974 set. 



The assumed accuracy of the scale used for the November 1974 photos was 

 verified by the actual ground measurements shown in Table A-1. Objects with 

 well-defined end points were selected without regard for elevation or orienta- 

 tion. The amount of error ranged from to -1.43 meters; the average absolute 

 error was 0.5 meter. Relative errors varied from -3.6 to 4.2 percent, averag- 

 ing 0.03 percent. 



Table A-1. Conparison between 



actual and ph 



otograph 



ic measurements. 



Object 



Approximate 

 elevation 



Distance 



(m) 



Error 



Relative 

 error 



1974 photos 



Actual 





Cm above LL) 







Cm) 



Cpct) 



Distance between 



1 



18.29 



18.66 



-0.37 



-1.98 



two groins 













Seawall 



1 



33.53 



32.92 



+ 0.62 



1.88 



Swimming pool 



12 



19.81 



19.72 



+0.09 



0.46 



Roof 



11 



27.43 



26.82 



+ 0.61 



2.27 



Seawall 



1 



49.38 



49.99 



-0.61 



-1.2 



Seawall 



1 



38.10 



39.53 



-1.43 



-3.6 



Seawall 



1 



35.05 



34.90 



0.15 



4.2 



Tennis court 



9 



45.72 



45.26 



0.46 



1.02 



Tennis court 



9 



36.58 



36.42 



0.16 



0.44 



Between lines in 



6 



18.29 



18.29 



0.0 



0.0 



parking lot 













Seawall 



1 



33.53 



34.14 



-0.61 



-1.79 



Seawall 



1 



29.26 



29.87 



-0.61 



2.04 



Roof 



9 



21.34 



21.49 



-0.15 



0.70 



Avg. of absolute 



values 







0.49 



1.66 



Avg. 



-0.13 



0.03 



68 



