pe 
FEE 
adi 
   
     
Fig. 2. Twinplex plotter with ER-55 projectors. 
Probably the most striking feature of the Twinplex is its simplicity. 
t, there are no moving parts to con- 
Once the model is set up in this instrumen 
tend with. Plotting is accomplished with the standard tracing table in the same 
manner as in the Multiplex and Kelsh systems. Because of the wide area cover- 
age, an 18-mile bridge for 20-foot contour accuracy can be accomplished with 
only 4 models (as compared to about 13 models for a 600 C-factor vertical sys- 
tem), thereby reducing control costs and bridging weaknesses. The repeat- 
ability of readings with the instrument is excellent and even the less experienced 
operator can duplicate his elevation readings accurately and easily. 
To verify the theory of the Twinplex and convergent photography, the 
Geological Survey has compiled an area of about 60 square miles using conver- 
gent photography taken at a flight height of 12, O00 feet. This compilation, when 
rigidly tested for vertical accuracy, showed a standard deviation of 3 feet. The 
average spot height reading error was H/6,000. The operator found that be- 
cause of the strong intersection of conjugate image rays and the increase in 
stereoscopic depth, the placement of 10-foot contours was relatively simple. 
While no definite conclusions can be drawn from only one test, the check 
of the initial Twinplex compilation substantiates the feasibility of convergent 
photography and Twinplex theory. Other governmental and private organiza- 
tions are known to be conducting similar investigations with very promising re- 
sults. 
nvergent photography and Twinplex 
There is no claim being made that co 
No one system or instrument can do 
will solve all photogrammetric problems. 
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