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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.
