13 
Deformation of turns with cross wind. The flight line spacing changes by A s 
3. Flying at a definite absolute height. 
Over known territory, particularly when 
several missions have to be flown on succes 
sive days, the correct level of photoflights is 
controlled by the electric altimeter. Water 
surfaces are particularly well suited for these 
measurements on account of their level plane 
of reflection and the increased reflection of 
electric waves. After the correct absolute al 
titude has been reached, the statoscope is 
turned on. When the aircraft is correctly 
trimmed, it can be easily kept on the correct 
level within a tolerance of +_ 10 rn? either 
with or without three-axis auto-pilot systems. 
On long-distance runs, the trim must be read 
justed from time to time, as there will be a 
gradual shift of the center of gravity of the 
airplane on account of the progressive dis 
charge of the fuel tanks. 
Practical Suggestions. 
Since photoflights are largely dependent on 
fair-weather periods and also because of high 
operating expenses, only an extremely care 
ful preparation of the flights will ensure sa 
tisfactory results. In addition to expert main 
tenance of the engines and the hull, particular 
attention must be given to the constant con 
trol of the calibration and expert installation 
of all navigation instruments including the 
automatic pilot system. 
For the navigational preparation of the 
flight, the territory to be photographed is 
subdivided into areas with equal relief, or, in