76 SURVEY NAVIGATION, CORTEN 
Sources of errors. 
| a. By suitable design and construction the instrumental error of the statoscope is limited 
to m = 1/30 mb only (equivalent to 30 cm approx.). 
b. The influence of the isobaric surface’s slope can be nearly eliminated if there is con- 
trol at the beginning and at the end of the strip. Linear interpolation is then possible. 
e. The residual errors are mainly due to the fact that a linear interpolation does not 
account for local and temporary variations in altitude of the isobaric surface. These 
variations are, under normal survey conditions, of the order of x 1-m. 
  
The above facts confirm the general experience in photogrammetry that statoscope 
height differences can be reliable to a standard error m — 251 5...2,0.m. 
  
Fig. 20. 
! 
Instrument s. 
Modern statoscopes record the measurements on the main camera's negative film. 
The Zeiss statoscope with electrical recording is well-known, whereas Wild has con- 
structed a new version. The U.S.S.R. statoscope supplies a continuous record, produced 
by the illuminated meniscus which is projected onto a continuously moving film. The 
difficulty of range limitation is overcome by using two statoscopes: number 1 is in actual 
operation and number 2 takes over at the moment that number 1 reaches its limit; this 
action is initiated by electrolytic contact at the liquid’s meniscus and is carried out by 
a servo mechanism, figure 20. I.G.N.’s statoscope will be described at the Congress.