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4* 
The discussion in this paper wil include the theory of convergent photo 
graphy, comparisons between new and old systems, the features of the 
Twinplex and adapted existing equipment, and the results of tests made 
using the convergent system. 
b) USE OF THE KELSH PLOTTER BY THE U.S. GEOLOGICAL 
SURVEY. 
Through a diversification of plotting instruments, the Geological Survey 
seeks to provide the means of reaching the most efficient solutions for a 
variety of problems in topographic mapping. Each instrument, used alone 
or in combination with other instruments, has a proper place in the over-all 
scheme of operations, but no single instrument provided all the answers. 
The Kelsh plotter is generally used by the Geological Survey for mapping 
areas of relatively low relief where the accuracy requirements are somewhat 
higher than those attainable in the Multiplex, but not as high as those attai 
nable in the universal plotters. A combination of Multiplex and Kelsh plotting 
can frequently be efficiently exploited by establishing the individual model 
control by Multiplex triangulation, compiling planimetry by Multiplex, and 
contouring by Kelsh Plotter. 
The low cost of the plotter makes it economically feasible to integrate 
the Kelsh plotter in a mass production mapping program such as that of the 
Geological Survey. A smaller organisation would doubtless use the plotter 
in a different way. 
Although some private users have reported good results using the Kelsh 
plotter as a high C-factor instrument the Geological Survey uses it conserva 
tively. Field tests of sheets compiled with the instrument have indicated 
performance of a higher degree of accuracy than anticipated, but the Survey 
is regarding these tests cautiously pending the accumulation of a sufficient 
volume of data for a conclusive evaluation. 
In the last few years the instrument has gone through a very rapid design 
evolution although the basic principles have remained the same. A final and 
completely satisfactory design has not yet been achieved; the Geological 
Survey is well aware of problems still to be solved. Nevertheless, the ins 
trument as developed thus far is highly satisfactory for the uses to which it 
is put in the Geological Survey. 
Although there are certain advantages in standarisation of equipment, 
the Geological Survey does not expect to settle on the Kelsh plotter or any 
other instrument as the basic apparatus for stereoplotting. Rather, it is 
intended to examine carefully each new development, foreign and domestic, 
and adopt those which fit advantageously into the broad program.