3 
the efficiency, accuracy, and convenience of use of both of these instruments. 
The improvements implemented to date have affected various components of the 
plotters, such as the projectors, the projector-support system, the measuring 
system, the illumination system, the electrical-control system, the image- 
distortion compensation system, and the viewing system. 
UNIVERSAL TABLE FRAME 
The support frame introduced by the Geological Survey in about 1952 
for use with ER-55 projectors (see figure 1) was also usable with the since- 
abandoned multiplex projectors. On the other hand, the Kelsh plotter (see 
figure 2), which was introduced extensively in Geological Survey operations 
at the same time as the ER-55 plotter, had virtually no components that were 
interchangeable with ER-55 equipment. 
A major improvement in plotting equipment in the Geological Survey has 
been the design and procurement in quantity of the universal table frame 
which is adaptable for use with the various anaglyphic double-projection 
systems as indicated in Table 1. 
Figure 3 shows the basic universal table frame equipped with pantograph, 
tracing table, granite reference surface, and electrical controls as used 
with the ER-55 projectors. The frame, consisting of two end supports joined 
by a welded box section and tubular cross members, gives sturdy support to 
the 4-inch thick granite slab. The reference or plotting surface of the 
granite is flat to plus or minus 0.025 millimeter maximum over the entire 
48-by-70-inch area. The central or working area of the granite surface does 
not depart from flatness more than plus or minus 0.013 millimeter. The 
frame-and-granite assembly is of such rigidity that no measurable disturbance 
of the stereoscopic model is ever encountered under normal operating 
conditions.