the distance of the points on the surface of the earth from the 
position of the aircraft at any given time can be determined. 
If the aircraft is flying a course parallel to the geoid, the 
profile of the earth's surface along the line of flight can be 
reconstructed by plotting the position of the aircraft on the 
abscissa and the measured distance of the ground points from the 
plane on the ordinate. In actual practice, the line of flight 
deviates from the isobaric surface and the isobaric surface 
itself is not exactly parallel to the geoid. The amount of 
deviation from the isobaric surface, however, can be measured 
with reasonable accuracy in the aircraft with a sensitive 
electronic aneroid. Similarly, the lack of parallelism between 
the isobaric surface and the geoid can be calculated by means of 
a formula and vertical control in the field. 
Aircraft path 
lsobaric surface p 
— — 
    
  
  
  
Fig: 1 
A mechanical process is being evolved so that the 
profile of the terrain can be recorded on a strip of graph paper 
as the aircraft flies over the ground. In this profile compensa- 
tion has already been made for the deviations of the aircraft from 
the isobaric surface; before making further use of the profile 
data, it is therefore only necessary to correct the lack of 
parallelism between the isobaric surface and the geoid. 
As one can see, the absolute elevation H of any point in 
the profile depends on two completely unrelated factors: the 
difference in the elevation between the aircraft and the initial 
point and the distance of the point in question from the aircraft. 
    
Hp = 24 * (+ A Zp % dz) = hy