This being the case, structural engineering often makes 
use of deflection measurements of the structural model for an 
indirect and much more reliable determination of design criteria. 
Dial indication gauges of 0.001' reliability are generally used 
in deflection measurements. In a few cases, gauges of 0.0001'' 
reliability are used. Obviously, the measurement of deflection 
should be rapid to reduce, as much as possible, the unfavorable 
effect of possible changes in the structure during the measure- 
ment. 
Photogrammetry is ideally suited for this purpose. The: 
use of photogrammetric techniques to measure structural defor- 
mations offer decided advantages over the direct (gauge) approach; 
most important of which are: 
a. A complete record of the displacements at all points 
of the model can generally be made in a fraction of 
a second. 
Not having to disturb the object by physical measure- 
ments. 
The photographs provide a permanent metrical record 
which can be consulted at any time for checking 
purposes or for additional measurements or information. 
Dynamic displacements can be measured by the use of 
rapid flash photography. 
Having no accessibility problems for the measurements 
themselves. 
Space frames, shells, and other ''complicated'' models 
present no added problems in the photogrammetric method. 
STEREOMETRIC SYSTEMS 
  
Data Acquisition 
  
The design of a photogrammetric system for data acquisition