AERIAL PHOTOGRAPHY 717 



general trend of the topography. This means they should be parallel to the main 

 rivers and drainage. In order to determine the altitude A above ground to fly, the 

 following formula may be used : S = A/f. This formula is practically the same in its 

 fundamental conception as the one used under the oblique section of this article. For 

 example, if a scale S is desired of 1 in. = 1500 ft. and if the camera to be employed is 

 of 8}4: in- focal length/, the altitude maj^ be determined as follows: 



1500 ft. = g^ (3) 



1500 ft. XSH = A (4) 



A = 12,375 ft. (5) 



A factor which may confuse the beginner is that there are two methods of expressing 

 scale. A scale may be expressed, for example, as 1 in. = 1000 ft. Another way of 

 expressing the same scale is 1:12,000 or 1/12,000. A scale stated in either of the 

 latter two ways may be reduced to feet per inch by dividing by 12. For example, to 

 determine the number of feet per inch when the scale is given as 1/20,000, divide 

 20,000 by 12, giving an answer of 1 in. = 1667 ft. 



In estimating the cost of a photographic flight, it is, of course, necessary to deter- 

 mine the number of exposures and the amount of fljang that will be involved. This 

 will depend upon the specifications as to scale, progressive overlap (which means the 

 amount that each picture must overlap the next consecutive picture), and strip 

 overlap (which means the amount that each strip of pictures must overlap the adjacent 

 strip of pictures). The specifications in most common use today call for a scale of 



90 finn ^^ ^^" ~ 1667 ft.) ±5 per cent, with the pictures taken so as to have 60 per 

 cent progressive overlap and 30 per cent strip overlap. 



If the strips must overlap 30 per cent and if the pictures are 9 in. wide, 30 per cent 

 of each picture, or 2.7 in., should overlap. This leaves a net width for a 9-in. picture 

 of 6.3 in. This 6.3 in. at a scale of 1667 ft. to the inch gives a distance of very close to 

 10,500 ft. as the separation between flight strips. 



If the specification further requires that the progressive overlap be 60 per cent 

 and if the size of the negatives is 7 in. in the direction of flight, then the overlap of each 

 picture will be 60 per cent of 7 in., or 4.2 in. Therefore, one picture must be taken 

 for each 7 in. minus 4.2 in., or 2.8 in. At 1667 ft. per in., a picture will thus have to 

 be taken every 4670 ft., approximately. 



We can now determine the number of pictures required to cover the area. By 

 laying the strips that the airplane will flj^ off on our map with a separation of 10,500 ft. 

 and measuring the length of each strip and dividing this distance by 4670, we can 

 determine the number of pictures required for each strip and by adding up the sum of 

 the strips, the total theoretical number of pictures for the area is determined. It 

 should be kept in mind that specifications usually require that the pictures cover a 

 certain amount beyond the actual boundaries of the area. It is the most common 

 practice to specify that at least 25 per cent of the width of the pictures must cover 

 outside the side boundaries of the job and that at least two picture centers must 

 fall beyond the boundary at the ends of each strip. Experienced organizations 

 mapping a large area generally take 25 per cent more pictures than the theoretical 

 number. In mountainous country the theoretical number is frequently' increased by 

 50 per cent in practice. Inexperienced personnel may shoot several times the theo- 

 retical number. 



In determiniiig the number of pictures which will be required to cover a given area, 

 variations in the elevations of the ground must be taken into consideration. If the 



