APPENDIX 



ANALYSIS PROCEDURE 



Various procedures are currently in use for making measurements from aerial 

 photos. Therefore, it is important to discuss the procedure and to estimate 

 the accuracy before attempting to understand the results. 



1. Photo Measurements . 



One commonly used procedure for taking measurements from aerial photos is to 

 make measurements from carefully selected low elevation reference points to the 

 feature being studied (Stafford, 1971). The reference point must be a clearly 

 defined stable feature which appears on all the sets of photos. After all the 

 measurements have been made and scaled according to each photo, changes can be 

 computed. The primary advantage of this method is that measurements are taken 

 directly from the photo. The disadvantage is that points of measurement cannot 

 be taken at equal intervals along the shoreline and it cannot be used in areas 

 with few cultural features; also, since measurements are made independently, 

 problems in defining the bluff line may caiose it to move lakeward, a physical 

 impossibility. This is particularly true when the time intervals are short and 

 the bluff line is relatively stable. 



A second procedure involves a form of indirect measurement where photo de- 

 tails are optically transferred to a base map by a device like a zoom transfer 

 scope (ZTS). Wilson and Everts (in preparation, 1980) provide an example of 

 this method. 



The ZTS allows magnification, rotation, and stretching of one image to 

 superimpose the image on another. It is, however, difficult and tedious to use. 

 For this reason, Istvan (1974) recommends that the ZTS should not be used to 

 determine shore and bluff recession because of optical errors, the difficulty 

 of matching photos precisely, and the greater potential for interpreter error 

 and fatigue. 



A combination of these procedures was used in this study to optimize speed 

 and accuracy. Measurements to the toe of the bluff and the shoreline were made 

 directly on each photo using an appropriately scaled grid of the reference line 

 and the station locations. This procedure was considered accurate enough to 

 identify shoreline and beach width variations. 



Greater accuracy was desired for the bluff line. Using an acetate overlay, 

 the bluff line on each photo was traced with the aid of a scanning stereoscope. 

 Using the ZTS, the photo with the bluff-line overlay still in place was then 

 superimposed on the corresponding November 1974 photo by matching the center 

 part of the photo. (The ZTS was only used to match the scales; the "stretch" 

 feature was not used.) 



After the photos were matched, the bluff line was transferred to an acetate 

 overlay on the November 1974 photos. In this manner, the bluff lines could be 

 compared as they were being drawn and errors due to photo interpretation were 

 reduced. 



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