HN 
  
Wernstedt-Mahan (Mahan for U. S. Geological Survey); and Multiscope 
(Spurr and Brown for Harvard University). 
OBJECTIVES OF U. S. INVENTORS 
A review of the literature describing the U. S, third-order 
instruments shows that all had very similar objectives, about as follows: 
1. Low cost, 
2. Easy portability. 
3, Use of contact aerial photographs. 
4, Good stereoscopic viewing of the model. 
5, Provision for drawing directly on the map manuscript, 
6, Correction of the scale variation due to relief in the model. 
7. Correction for nominal tilts and differences in flight altitude 
in the aerial photography. 
8. Reasonably accurate delineation of the horizontal position of 
planimetric detail and contours. 
UTILITY OF THE U. S, THIRD-ORDER INSTRUMENTS 
As mentioned above, these U. S. instruments were designed to 
meet a specific problem or need. In early use, all were quite successful; 
however, in nearly every instance of their use for routine topographic map 
production, they have been superseded by more costly and higher order 
instruments. Without doubt the brevity of the productive-liife. of this 
equipment was brought about by the advent, ready availability, and low 
cost of the U. S, multiplex and later the Kelsh and Balplex plotters. 
DEFICIENCIES OF U. S. THIRD-ORDER INSTRUMENTS 
Since the third-order instruments did not survive in mapping 
competition, we must conclude that either the instruments did not come up 
to the inventors' expectations, or that the inventors did not correctly 
anticipate the requirements for the instruments. A quick review of the 
inventors! objectives above will show that their "requirements" were 
quite demanding. The experienced critic will be quick to point out that 
the U, S, inventors were unduly optimistic in hoping to get all of the 
qualities listed from a third-order instrument, It is therefore quite 
understandable that the instruments did not perform as hoped. 
It appears that the early U. S. inventors did not fully appreciate 
the economics of map production. They placed undue emphasis on the use 
of low-initial-cost instruments in the mapping procedure. Since the cost 
of the manuscript map product is the sum of the costs for aerial photogxaphy, 
ground control, photogrammetric map production and the field edit, and 
since all of these factors are interrelated, the initial cost of the photo- 
grammetric instrument is only one of many items which contribute to the 
final cost. It is quite conceivable, that the initial cost of these 
photogrammetric instruments is of minor importance in determining the 
map manuscript cost. We are all acquainted with situations where high- 
cost, high-order photogrammetric equipment can be used to achieve the 
lowest cost product. 
FUTURE OF THIRD-ORDER INSTRUMENTS 
A review of past experience shows that third-order instruments 
have only a limited application in routine U, S, photogrammetric mapping.