orrection for 
uipment* 
rican Multi- 
based largely 
eratively by 
'ompany and 
pervised first 
er by-R. K. 
beginning in 
e resulted in 
tive printers 
rection, and 
iation of the 
Is of interior 
he projectors 
ed range and 
desired scale, 
  
51 model. 
PHOTOGRAMMETRIC ENGINEERING 11 
Kelsh Plotter 
The basic principles of this instrument, 
Figure 14, were conceived by H. T. Kelsh 
in 1943, prior to his association with the 
Geological Survey, and were developed 
by him and other members of the Survey's 
technical staff beginning in 1947. This plot- 
ter has passed through an extremely rapid 
metamorphosis. The basic improvements 
over previous double projection plotters 
have, however, remained the same, namely 
(1) the swinging light source, (2) correction 
of camera lens distortion through adjust- 
ment of the projector principal distance by 
means of an arm and cam arrangement, 
and (3) contact-size diapositive plates. 
In 1951, an experimental adaptation of 
the Kelsh plotter was constructed by the 
Trimetrogon Section to demonstrate the 
possibility of its use with oblique trimet- 
rogon photographs. 
  
Fic. 15. Twinplex prototype. 
The Twinplex? 
The prototype Twinplex stereoplotting 
instrument, Figure 15, developed by R. K. 
Bean, was completed in 1950. Tt is designed 
to utilize low-oblique, wide-angle photog- 
raphy obtained with two synchronized 
and rigidly coupled cameras. (The two 
cameras may be aligned either along the 
flight line for precision mapping, or trans- 
verse to the flight line for reconnaissance 
mapping. Two unrectified diapositives, 
corresponding to the two exposures made 
at one camera station, are used in a set of 
twin projectors. Each exposure must be 
paired with an exposure from an adjacent 
camera station in order to produce the 
stereoscopic model. When either of the 
twin projectors has been oriented in pro- 
jecting position, the other projector will be 
correctly oriented when it in turn is swung 
into projecting position. 
Stereodetailer 
This instrument, Figure 16, devised by 
E. I. Loud, Jr., in 1951, is used in compila- 
tion review to transfer certain detail from 
aerial photographs to multiplex manu- 
scripts. It applies the principle of the 
sketchmaster to a simple lens stereoscope. 
The instrument can be visualized as a 
sketchmaster in which the map and photo- 
graph have exchanged positions, with a 
left eyepiece added so that another photo- 
graph can be viewed with the left eye to 
give a stereoscopic image. Only one eye 
sees the superimposed image, with the 
effect that the lines of the map give the 
illusion of relief, seeming to conform to the 
relief in the photo model. No provision is 
made for tilt, but this 1s not necessary 
since it is intended to position only small 
areas at a time to the abundant detail of 
the completed manuscript. Enlargement 
ratios ranging from two times to five times 
can be obtained by changing the distance 
between the eyepiece and the large mirror, 
and inserting a suitable lens at the eye- 
piece. 
     
    
+ 
520 
Fic. 16. Stereodetailer.