The next problem is that of compiling a map from the an- 
notated aerial photographs. Where no previous base map exists 
a planimetric map may afford sufficlent control for geologic 
compilation. One of the simplest methods of obtaining a plani- 
metric map is by use of vertical aerial photographs.  Photo- 
graphs of the Maverick Springs quadrangle are used to demon- 
strate the radial-templet method of triangulation. Four points 
of horizontal control are plotted at 1:24,000 scale on a 
polyconic projection, which serves as a base upon which slotted 
cardboard radial templets are assembled. The locations of 
photograph centers and pass points as intersected by the tem- 
plets are then marked on the base. Each of the trainees 1s given 
a copy of this base and compiles a planimetric map using the 
radial planimetric plotter. The finished maps are compared to 
the standard topographic map and errors diagnosed. The photo- 
graphs used have very slight tilt and the compensation for 
relief displacement and scale differences among the photographs 
when using the radial planimetric plotter is adequate so that a 
careful operator can produce a map of surprising planimetric 
accuracy. À demonstration of the methods used to compile the 
geologie data as annotated on the vertical photographs directly 
to a topographic base map is given. Where relief is very low 
a vertical sketchmaster is used; where relief is greater the 
radial planimetric plotter or other orthographically plotting 
instruments are used, 
At this point in the training program the trainee has 
become aware of many of the considerations that enter into map 
compilation from aerial photographs and he has sufficient back- 
ground to understand the advantages of the various higher order 
projection-type plotters--Kelsh, Multiplex and Balplex.  Inas- 
much as thorough knowledge of these instruments is gained only 
after lengthy experience in operation, the instructor sets up 
the stereoscopic models thereby permitting the trainees to 
spend all of their training time in familiarization, operation 
of the tracing table, and working out three-point dip and strike 
problems, as well as making geologic interpretations from the 
three-dimensional model. An interesting observation voiced by 
almost all of the trainees is that they would like some method 
of delineating geology in the stereoscopic model, This could 
be accomplished by working with annotated diapositives,  How- 
ever, the annotated lines are enlarged and obscure critical 
detail in the stereoscopic model thus preventing the geologist 
from taking full advantage of the accuracy potential of the 
plotter, Experienced operators have found that by periodically 
viewing properly annotated contact prints of the model adequate 
geologic orientation is maintained while compiling with the 
projection-type instruments. Further, in many instances the 
operator who 1s familiar with the geology can work with the 
projected model alone and achieve excellent results, These 
facts are mentioned as they are important considerations if 
we are to encourage geologists to use projection-type plotters,