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.