Lor 
.d 
e 
N 
considerably the density of horizontal ground control (1). 
However, this is not the case in vertical block adjustment. It 
is not possible to eliminate vertical ground control to the same 
extent. Therefore, auxiliary data which would allow a comparable 
reduction in vertical ground control are extremely important. 
The auxiliary instruments listed in Table I have been des- 
cribed in many text books and technical publications. I will, 
therefore, confine my remarks to a brief discussion of their ad- 
vantages and limitations, as well as the accuracy of the data 
determined by each instrument. 
Both the Airborne Profile Recorder and the statoscope use as 
  
reference a barometric pressure surface. The height corrector in 
an APR is, in fact, a statoscope. Under favourable conditions, 
and when great care is taken in securing a proper source of static 
pressure, the relative accuracy of differences in elevations be- 
tween successive exposure stations is in the order of + 1 meter. 
A relative accuracy for the entire length of a strip, after appli- 
cation of Henry's correction would be in the order of + 1.5 to 
+ 2 meters. This accuracy however, cannot always be achieved 
under operational conditions and one must be prepared to accept 
discrepancies of + 2.5 to + 3 meters. They will however, be con- 
fined to a small percentage of the data. The above refers to 
the height corrector (statoscope) and not to the total APR system. 
The other component in the APR system is an instrument for 
measurement of the distance between the aircraft and the ground. 
This distance is called the "terrain clearance". Whether the