Theory for the determination of differences in elevation 
of points on the aerial photograph by measuring the x-parallax 
is next studied. The various parallax measuring instruments 
are discussed and demonstrated, After learning the fundamentals 
of operating the various height finders, parallax bars, or stere- 
ometers, the trainee works with a set of photographs and the ac- 
companying topographic map. Differences in elevation of given 
points are determined and compared with the differences as rep- 
resented on the topographic map. When the trainee has become 
adept in this technique the effect of tilt in the stereoscopic 
model is analyzed, using the technique in conjunction with the 
topographic base map. The parallax bar 1s then used to make 
three-point dip and strike determinations and results are com- 
pared to measurements previously made in the field with the 
Brunton compass or plane table and alidade, Measurements of 
the thickness of stratigraphic units are also made, using the 
parallax bar. 
Vertical exaggeration is demonstrated by using the Stereo- 
slope Comparator, This instrument, developed by the U, S. Geo- 
logical Survey, 1s designed to compute the inclination of planar 
features in a stereoscopic model using as its principal of opera- 
tion a vertical exaggeration constant for a given operator under 
Specific viewing and photographic conditions. After thorough 
orientation in operation of this instrument the geologists 
measure the strike and dip of strata as expressed in the aerial 
photographs and compare the results with measurements of the same 
features performed by field methods. By this demonstration the 
trainee is made aware of the effects of vertical exaggeration 
and why they must be considered by the photo-interpreter and 
specifically by the geologist whose work depends on correct in- 
terpretation and visualization of dipping strata. 
Two methods for properly annotating the aerial photographs 
are shown, 1) annotation of the entire area of alternate pho- 
tographs in a flight line and, 2) annotation of the right half 
of every photograph in the flight line. The latter method 
permits a continuity of lines from one stereoscopic model to 
the next.