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HANDBOOK OF PHOTOGRAPHY 



variations in ground elevation are not sufficient to require that the area be broken 

 down into elevation zones, the elevation at which the airplane will fly is generally 

 calculated from the mean elevation ::f the ground, in which case the overlap, both 

 progressive and strip, must be calculated for the maximum elevation of the ground 

 (Fig. 7). Thus, if the desired scale is 1 in. = 1500 ft. and the theoretical airplane 

 altitude is 12,375 ft. above a mean ground elevation of 800 ft. above sea level, the 

 airplane must fly at an elevation of 13,175 ft. above sea level. Now, if there are hills 

 going up to an elevation of 1300 ft. above sea level, the scale for the flight should be 

 figured as follow^s: 13,175 ft. - 1300 ft. = 11,875 ft. 



With a focal length of 8V4 in., this gives a scale of approximately 1 in. = 1440 ft., 

 rather than the theoretical scale of 1 in. = 1500 ft. The scale of 1 in. = 1440 ft. must 



f= Focal length of camera 

 A= Al+i+ude 

 S= Scale 



1<- fm//e -A 

 M, 



Fig. 7. — If the plane does not fly over level ground, the scale of the photograph will not be 

 uniform, and corrections will be required. 



be used in laying out the separation betw^een strips and determining the progressive 

 overlap of the pictures. 



After the flight has been planned and the flight map prepared, flying may be under- 

 taken. The pilot should so plan his period of climbing that he is over the area to be 

 mapped as much as possible. During this time he should study the area in relation 

 to the map, locating the boundaries so that when the mapping actually starts, he will 

 have these boundaries firmly in mind. He should also pick out prominent topographic 

 features lying in the prolongation of the boundary, marking the beginning and ending 

 of the strips. He will frequently find that there is nothing exactl}^ under him by 

 means of which to determine the boundary, and he will often be dependent upon 

 aligning himself in between the two distant objects. As the airplane approaches the 

 mapping altitude, the outside temperature should be noted. This temperature should 

 normally decrease approximately 3°F., or 2°C., for each 1000 ft. of increased elevation. 

 If the air temperature varies considerably from this theoretical gradient, the altimeter 



