U.S.A.-4 PHOTOGRAMMETRIC ENGINEERING 
plotter with vertical photography, ER-55 plotter with vertical photography, 
heavy plotter with vertical photography, and ER-55 plotter with convergent 
photography. Keeping in mind that values shown are relative rather than 
absolute, there will first be compared the layouts for the ER-55-with-conver- 
gent-photography system and the heavy-plotter-with-vertical-photography 
system, as the latter is usually regarded as the most economical of the con- 
ventional systems with respect to supplemental control requirements. 
It can be seen from the figure that the area of an ER-55-with-convergent- 
photography model is more than twice that of a heavy-plotter-with-vertical- 
photography model although both are planned to yield the same accuracy of 
results. It takes only 2.5 ER-55-with-convergent-photography models to span 
the 15-minute quadrangle, compared to 5 heavy-plotter-with-vertical-photog- 
raphy models. The prospect of performing vertical bridging successfully is, of 
course, much better for a 3-model (or shorter) bridge than for a 5-model bridge. 
A strong feature of a 3-model bridge is that there is only one uncontrolled 
model between two controlled models, as is readily apparent on the diagram. 
Only 10 models are required for coverage of the quadrangle by the ER-55- 
with-convergent-photography system, compared to 20 for the heavy-plotter- 
with-vertical-photography system. Each system requires 10 H-V (horizontal 
and vertical) control points. If vertical bridging is used, none of the V-points 
shown on the diagram are required. If the V-points must be obtained in the 
field, 20 are required for the heavy-plotter-with-vertical-photography system, 
compared with 10 for the ER-55-with-convergent-photography system. 
Differences of the same nature, but successively greater in degree, are 
apparent when the ER-55-with-convergent-photography system is compared 
with the other systems shown on the diagram. 
CAPITAL OUTLAY 
It would be a fair question to ask why the use of heavy plotters with con- 
vergent photography is not included in the foregoing analysis. There is every 
reason to conclude that convergent photography used in a heavy plotter capable 
of accommodating it would result in a system as efficient, operationally, as the 
ER-55-with-convergent-photography system, and possibly more so. But from 
the economic standpoint, the question of capital outlay must be considered. 
To replace present Multiplex by ER-55 equipment will cost the Geological 
Survey less than $3,000 per unit, including a prorated cost for Twinplex aero- 
triangulation equipment, diapositive printers and other accessories. This low 
unit cost applies because the existing Multiplex table frames, slate table tops, 
supporting frames, tracing tables and auxiliary equipment are utilized with 
the ER-55 projectors. In essence, the ER-55 projectors replace the Multiplex 
projectors on existing Multiplex units. In addition to the factor of low unit cost, 
it is important to remember that fewer units are required, in a ratio of 2 to 3, 
as already mentioned, for the same amount of production. 
On the other hand, heavy plotters would cost on the order of $25,000 to 
$60,000 per unit. Consider what the capital outlay would be for installing heavy 
plotters as the work-horse instrument as compared to the capital outlay for 
ER-55 plotters. In an organization like the Geological Survey, which operates 
hundreds of plotting units, this difference in capital outlay would run into mil- 
lions of dollars. We know of no operation anywhere, in which heavy plotters 
are concentrated in great numbers for the execution of a really extensive map- 
ping program. This does not mean that the heavy plotters do not have an im- 
portant place in photogrammetric operations. The Geological Survey has nine 
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