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by Harold J. McMillen 
US, 
Geological Survey 
ABSTRACT 
USE OF THIRD-ORDER PLOTTING INSTRUMENTS IN THE UNITED STATES 
Third-order instruments, which provide for an approximate and 
direct orientation of aerial photographs, were first manufactured in the 
United States during the early 1940's, 
Thelr conception arose from the 
needs of small groups of photogrammetrists whose immediate mapping 
problems served as the principal objectives for design. 
Early use of these plotters was successful with respect to the 
needs for which they were originally designed; however, the introduction 
of low-cost, higher order U. S. plotters (multiplex, Kelsh, and Balplex), 
together with tighter restrictions on photogrammetric mapping accuracy, 
made their extensive use in compilation phases undesirable. 
The inability 
of the third-order plotters to meet the competition of the more advanced 
plotters was a direct result of the limited objectives of early U, S. 
inventors. 
secondary objectives were never attained, 
Because of the emphasis on lqw initial instrument cost, many 
The future does not necesserily exclude a possible and justifi- 
able use for these specific-purpose plotters. 
At the present time, the 
U. S. Geological Survey is engaged in extensive testing of these instru- 
ments to determine whether their capabilities are sufficient for applice- 
tion to map-revision phases. 
In summary, the characteristic qualities of third-order plotters 
are low initial cost, portability, and direct use of paper prints. 
These 
qualities are desirable, and perhaps sufficient when the objectives are 
limited; but they become less significant when high-grade results are 
desired since a relatively small additional investment enables the user to 
provide himself with an instrument that gives much more satisfactory results. 
  
Third-order instruments are generally defined as those simple 
instruments which provide only an approximate orientation of the photographs. 
They usually do not afford adequate compensation for image displacement 
caused by tilt and relief, 
Their accuracy depends upon conditions--near 
vertical photography and flat terrain--which are seldom encountered in 
practice. 
During the early and middle 1940's a number of third-order 
instruments were designed, manufactured, and used in the United States. 
It should be noted that these instruments were not designed 
instrument manufacturers to fit third-order requirements in 
Rather they were the developments 
or small groups of photogrammetrists who were attempting to 
specific and practical mapping problem. 
Army); KEK (King, Elliot, Kail for U, S. 
field of photogrammetry, 
(Cook for U. S. 
Some of these were: 
by professional 
the general 
of individuals 
solve some 
Stereotopograph 
Forest Service): 
"Presented at Ninth International Congress on Photogrammetry, London, 
England, September 1960. 
2 Publication authorized b 
xp 
/ 
the Director, 
U. 
S. Geological Survey.