(512) 
or structure, but which cannot be correctly interpreted from a study of the 
photographs alone. 
Field geologists and other persons using vertical aerial photographs 
generally avoid photogrammetric dip calculations on them because of a distrust 
of the accuracy obtainable. It is true that a small amount of tilt may so affect 
the calculated dip as to give misleading results, but the tilt present in most 
vertical aerial photographs rarely exceeds 1 degree. Accordingly, dip calcula- 
tions made by assuming the photographs to be truly vertical will not be 
seriously in error. New techniques in aerial photography are expected to 
eliminate any appreciable tilt (more than 1 degree), and precise dip calculations 
can then be made. 
The simple procedures for dip calculations are described in sufficient 
detail so that any worker familiar with aerial photographs can calculate the 
angle of dips or slopes without having had prior photogrammetric experience. 
The basic requirement for making dip calculations on vertical aerial 
photographs is the presence of a recognizable dip slope or a bedding trace on 
irregular topography. Points “u” (the upper point on a dip slope or bedding 
trace) and “1” (the lower point of a dip slope or bedding trace) are terms used 
throughout this paper. These two points do not have to be on the line of dip 
from each other for dip calculations. 
A source of error inherent in measuring the angle of dip slopes is the 
assumption made by the geologist that the hill slope and bedding plane are 
coincident. Actually the lower part of the slope may be covered with talus, 
causing the slope to have a smaller dip than the underlying strata. Although 
this error cannot be adjusted photogrammetrically, it is generally small if the 
geologist is discriminating. 
Dip calculations presented here are based on the geometry of the camera 
lens and the photographic film, and not on the geometry of the airplane and 
the ground. Consequently, it is not necessary to know the scale of the photo- 
graph, nor the height of the airplane above the ground, nor the absolute 
elevation of the top or bottom of the dip slope, nor the absolute elevation of 
any point on the grcund. Moreover, this information generally is not available. 
Instead, photo measurements of horizontal distances which have been corrected 
for displacement, together with the focal length (f) of the camera lens, and a 
measurement of the difference in parallax (dp) are used. 
In this paper, upper case letters in equations refer to points on the ground; 
lower case letters refer to corresponding points on the photograph. All measure- 
ments should be made in the same units, and are specified in millimeters for 
photo points and in feet for ground points. 
The writer wishes to thank Dr. W. H. Mathews of the University of 
California under whose guidance this method was developed. Grateful acknow- 
ledgment is made to E. R. Goodale of the Creole Petroleum Corporation for 
his helpful criticism during the early stages of preparing this paper, and for 
his technical suggestions toward the further development of the transparent 
overlay method. Dr. R. N. Colwell who was teaching at the University of 
California is thanked for his critical reading of the original manuscript. 
  
  
  
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