ap 
late 
aphy, 
to- 
It is also the author's opinion that unless there is some miraculous and 
unforeseen development in these instruments, they will not have a prom- 
inent place in routine mapping in the future. What then is the role 
of the third-order instruments? 
The author believes that the third-order instruments may have 
a place as special- and limited-purpose instruments. For example: for 
the intensification or transfer of geologic intelligence, or for the 
addition or revision of planimetric detail in up-dating our topographic 
maps. The U. S. Geological Survey is now actively investigating this 
latter use. The investigation is directed toward determining the 
instrument requirements for performing the work and the characteristics 
and capabilities of the third-order instruments which are now available. 
INSTRUMENTS FOR MAP REVISION 
Some of the clear requirements for a revision instrument are: 
1. Reasonable cost (it must be considerably less in cost than 
the second-order instruments capable of more accurate mapping). 
2. Easy portability, 
3. Use of contact aerial photographs. 
4, Good stereoscopic viewing of the model (the viewing system 
should magnify the photo image 2-4 diameters). 
5. Provision for drawing directly on the manuscript (at a pre- 
determined manuscript scale which may be either an enlarge- 
ment or reduction of the photo scale). 
6. Reasonably accurate delineation of the horizontal position of 
planimetric detail. (Even when using aerial photographs having : 
small tilts and having modest relief). . 
7. Accommodation of photographs of 9" x 9" format or smaller, which 
are produced by the commonly used aerial camera with F.L. be- 
tween 88 and 305 mm. 
8, Simple and convenient operation; it should be possible for the 
operator to view the stereo model and the map manuscript without 
changing his position. 
It may be noted that requirements 1-6 are identical with the 
"Objectives of U. S. Inventors" listed above, except that contouring 
capability is not expected. Also, it is interesting to note that 
except for the first three items--cost, portability, and use of contact 
prints--which do not relate to instrumental capability, the remainder of 
the requirements are very nicely fulfilled by any of the first-order 
universal instruments. Thus, it is evident that we are near to the danger 
which overtook previous designers and users of third-order instruments, 
that of expecting third-order cost and first-order results. 
CONCLUSI ON 
In summary, the limited application of third-order plotters 
seems to be a matter of simple economics. The only sources of economy 
with these instruments are low first cost and the use of paper prints 
instead of glass diapositives. Over a long period of use, these 
economies become insignificant in comparison to the losses arising from 
the lack of ability to produce a better product. Theuser: soon finds that 
by making a relatively small additional investment (that is, it is small 
when amortized over the life of an instrument) he can provide himself with 
an instrument which produces much more satisfactory results. 
2 76699 = 60