jgerpendicularly to the 
‘or this print is difficult 
in the display was not 
ratus was not available. 
xactly parallel and are 
y simplified scheme for 
cture. This view varies 
ates can be varied but 
be varied they will be 
jn in scale the oblique 
of measurement that is 
rith the Bench Camera 
complementary to the 
ventional camera. The 
ind and below the ob- 
h have been made with 
ment as a shadow pro- 
ne step, by silhouetting 
[he photographic plate 
wide application in the 
tographic standpoint, it 
grapher to present ter- 
tain scale accuracy. In 
wuld be of considerable 
t have further applica- 
principles allow for the 
observer from an infi- 
in addition, possess the 
in all three dimensions. 
(511) 
PHOTOGRAMMETRIC DIP CALCULATIONS !) 
by 
D. H. Elliott ?). 
Abstract. 
A method of calculating the angle of inclination of hill slopes and of strata 
on vertical aerial photographs, as developed in this paper, requires no instru- 
ment other than a pocket stereoscope. Besides the stereoscope, only a sheet of 
transparent acetate and a millimeter scale are needed. The calculations gene- 
rally have an accuracy to within plus-or-minus 10 percent of the value observed 
in the field (except possibly for dips approaching horizontal or vertical) as 
checked by a small number of surface observations. 
All calculations are based on the geometry of the camera lens and the 
photographic film. Consequently, in order to calculate the dip, it is not neces- 
sary to know the scale of the photograph, nor the height of the airplane above 
the ground, nor the absolute elevation of the top or bottom of the dip slope 
or bedding trace, nor the absolute elevation of any point on the ground. This 
is possible because photo distances are proportional to corresponding ground 
distances if both are related to a common datum. | 
A line of profile across the photographs can be drafted using a variation 
of the transparent overlay method. 
I. Introduction. 
Dip calculations may be made on vertical aerial photographs without the 
use of any instrument (except the pocket stereoscope) using methods developed 
in this paper. In addition to the pocket stereoscope, a sheet of transparent ace- 
tate and a millimeter scale are needed to make the photo measurements neces- 
sary to solve for the angle of dip. No calculations need be made to solve for 
the angle of dip if a nomogram is utilized. 
The methods presented here are especially useful for geological studies in 
measuring the inclination of strata for structural interpretation — either from 
bedding traces or dip slopes — and for soil erosion, forestry, engineering, and 
military studies in measuring the angle of hill slope. To simplify discussion, the 
term “dip slope” is used for a hill slope whether or not this slope is controlled 
by a bedding plane; the term “strike” is used for the direction of a horizontal 
line on this slope surface; and the term “dip” refers to the inclination in the 
direction of steepest slope. 
All photogeologic interpretation involving dip calculations, lithology, or 
structure must be checked in the field. This is essential, for in many cases 
interpretation is based on topographic expression simulating normal lithology 
  
1) In partial fulfillment of the requirements of the Master’s degree at the University of Cali- 
fornia. Manuscript submitted for publication February 1951. Published in full by State of 
California, Division of Mines, Special Report 15, “Photogeologic interpretation using photo- 
grammetric dip calculations.” 
2) Photogeologist, Stanolind Oil and Gas Company, Casper, Wyoming.