Camera 



-dR 



Ground 



Nadir 

 Figure A-1. Effect of relief displacement. 



An estimate of the probable error in the radial displacement between the 

 reference line and the bluff line at a station was computed for the average 

 quantities given above and was found to equal only 0.23 meter. This was for 

 a single measurement and the actual effect for successive measurements can be 

 assumed to be considerably less. Because the bluff line was usually displaced 

 west from the nadir, relief displacement is always a positive effect and un- 

 like scale variations, it cannot be assumed to follow a normal distribution. 



A major source of error which is difficult to quantify is that resulting 

 from interpretation and human error. These errors occur from improper inter- 

 pretation of the various beach and bluff features, improper photo matching, 

 and careless measurements. Photo interpretation is a tedious process and 

 should be recognized as such. Errors can be minimized by careful photo selec- 

 tion, by using the siiiplest possible procedure, and by including frequent checks 

 and remeasurements. 



Analysis errors resulted from inaccurate bluff-line identification and trans- 

 fer and from the accuracy of the measuring device. Because all bluff measure- 

 ments were made on the November 1974 photos, the errors are independent of scale 

 variation errors. Moreover, since the errors are random, it can be assumed that 

 they have a zero mean and that the amount of error follows a normal distribution. 

 These assumptions allow the standard deviation of the error to be estimated by 

 following the procedure previously used for scale variations due to tilt and 

 relief. 



71 



