APPENDIX A 



DETERMINATION OF EFFECTIVE AREAS ON AERIAL PHOTOS 



Delineation of effective areas on aerial photos requires marking off the boundaries of the 

 area on each photo that appears on that photo alone. Overlap occurs perpendicular to the flight 

 line and between lines of flight. The procedure is as follows: 



Marking boundaries perpendicular to the flight line - -endlap . 



Step 1. - -Taking the first two photos in a flight line, place photo 1 over photo 2 so that 

 images common to the two photos are approximately superimposed. Then, rule a straight line 

 (in ink or crayon pencil) on photo 1 so it approximately bisects the area of endlap and is approx- 

 imately perpendicular to the line of flight. This line should pass through two easily recognized 

 image points, several inches apart on the photos, and representing the highest points of topog- 

 raphy in a position suitable for the ruled line. 



Step 2. - -Duplicate this ruled line on photo 2 by drawing it through the images of the same 

 points on this second photo. This duplication is simpler if the photos are examined in stereo - 

 vision. If both lines pass through image points representing high points in the topography, then 

 no photo detail will be excluded. Some detail at low elevations may be duplicated within the 

 effective area of the two photos, but if ruled lines pass through high points, effective areas can 

 easily be separated and adjustments made under stereovision. 



Step 3. - -Repeat this procedure for all photos in a single flight line, pairing photo 2 with 

 photo 3, 3 with 4, etc. 



Marking boundaries between lines of flight --sidelap. 



Step 1 . - -Place the first photo in one flight line over the first photo in an adjacent flight 

 line so that images common to the two photos are approximately superimposed. Then, rule a 

 straight line on the top photo so it is approximately parallel to the flight line and approximately 

 bisects the area of sidelap. If possible, this line should pass through two prominent image 

 points. Draw this sidelap boundary only until it meets the endlap boundaries drawn previously. 



Step 2. - -Duplicate this ruled line on the bottom photo. Since stereovision of images from 

 adjacent flight lines is difficult or impossible, duplication of the ruled line must be guided by lo- 

 cation of image points crossed by the line on the top photo. It is convenient to hold the two photos 

 on a desk top in overlapped position with one hand. The other hand can be used to pull up one 

 edge of the top photo so that images common to the two photos can be identified and marked. 



Step 3 . - -Repeat the procedure for all other sidelapping photo pairs. The boundaries 

 ruled for each endlap and sidelap area wUl define the effective area on each photo. 



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