AERIAL PHOTOGRAPHY 719 



will be considerably in error. For example, suppose the pilot takes off with a ground 

 temperature of +20°C. and flies to an indicated altitude of 15,000 ft. If upon reach- 

 ing this altitude the temperature has dropped only to -i-10°C., the altimeter will read 

 15,000 ft. when the true altitude is about 15,500 ft. Great variations in the temper- 

 ature gradient do not frequently occur but are present often enough to justify the 

 exercise of considerable caution. This is particularly true when flying over hot 

 desert regions or under conditions of extreme winter cold. The following empirical 

 formula may be employed for approximate corrections. If the quantity in the bracket 

 comes out minus, the altitude is too high, descend; if plus, ascend. The correction 

 is given by: 



ft. 



where A = indicated altitude from altimeter; 



T„ = temperature on the ground one hour after sunrise, plus 2°C. for each 1000 



ft. of ground elevation above sea level; 

 Ta = temperature aloft (Centigrade, outside of cabin in free air). 

 If quantity in bracket comes out +, ascend. 

 If quantitj'^ in bracket comes out — , descend. 



Example: 



A = 15,000 ft., field at sea level 



r, = -f3o°c. 



Ta = -MO°C. 

 15,000 



15,000 



30 - 30 - 10 



500 



500 

 = 30 [30 - 30 - 30 - lOj 



= 30 (-40) 

 C — —1200 ft. (minus sign, you are too high, go down) 



Upon reaching the desired altitude, the pilot should trim ship and start down the 

 course at a constant air speed following landmarks or in accordance with the compass 

 reading. In the beginning of this strip, he should line up at some distance ahead two 

 easily recognized landmarks which can be used as sights for checking drift. If there 

 is a cross wind, he will note almost immediately that he is drifting off this imaginary 

 line, and he should at once commence to crab the ship into the wind experimentalh- 

 until he finds the desired amount of crab to carry him along the predetermined course. 

 Having determined the proper amount of crab, he should continue along this course 

 while the photographer turns his view finder the correct amount to compensate for 

 the crab of the airplane and makes the necessary time studies (if he is going to operate 

 by time) to determine the interval between exposures. When the photographer has 

 completed his determinations, the pilot should turn around and backtrack on the 

 course following the same procedure in order to determine the time interval on the 

 back course. As soon as the photographer has completed his studies on the back 

 course, they are ready to start mapping. The pilot should settle down on the course 

 2 or 3 miles outside the boundary, and the photographer should get settled in a 

 comfortable position from which he should move as little as possible, as his movement 

 will affect the trim of the ship. There is some difference of opinion as to whether the 

 pilot should instruct the photographer as to when to start photography, or vice versa. 



