and L (fig. 2) may be 
uation I) in cases where 
the dip slope or bed- 
A 
tions presented in this 
"nce. 
plane, the upper point 
he difference in height 
r this displacement by 
methods described in 
btained is the distance 
on of u. The relation- 
positions of u and I is 
Dip Calculations. 
calculations. In com- 
e photograph is gener- 
ative tilt between two 
iriables enter into the 
he direction of tilt of 
h other is significant. 
the effect will be less 
| effect when the direc- 
ctions. Nevertheless, a 
culations, especially as 
n of the line of flight, 
direction of tilt. Both 
1ese relationships have 
tail in section IV A6. 
Iowever, this involved 
low an approximately 
ning Air Stations. 
the two air stations is 
ground, and has little 
n be corrected at the 
| measured photo base 
int is small (generally 
(517) 
less than 2 degrees), except in those cases where the difference in elevation 
between the transferred center and the point is considerable. However, the 
simple correction as described in section IIIB should always be applied to attain 
the highest accuray possible. 
4. Improper Alignment of the Photographs. 
This applies only to the measurement of the differences in parallax using 
the height finder. If. the two photographs of a stereo pair are not properly 
oriented in regard to the line of flight, this measurement will be worthless. 
Because parallax, by definition, is measured in the direction of the line of flight, 
any apparent difference in parallax measured on incorrectly oriented photo- 
graphs does not represent the true difference in parallax. Moreover, a slight 
error in alignment may give a relatively large error in the measurement of the 
difference in parallax. 
5. Lens Distortion. 
This factor has no appreciable effect on measurement made to the closest 
0.01 millimeter (as in the case of the difference in parallax) on the photographs. 
6. Negative and Paper Distortion. 
Distortion of properly-handled negatives is negligible. However, distortion 
of untreated, single-weight positive paper prints may be of sufficient magni- 
tude to affect direct measurements on the photograph. To avoid this, a non- 
distortion paper which is waxed on the back to prevent moisture penetration 
should be used. 
7. Incorrect Direction of Strike and Dip. 
It is essential that the geologist be able to recognize dip slopes and bedding 
traces. If the strike and dip are plotted carefully, the difference between the 
plotted directions and the true direction generally will be insignificant, except 
possibly in cases of low dip, or in cases where long strike or dip construction 
lines are necessary (section IV A4). 
8. Accuracy Possible in the Measurements. 
Measurements scaled on the photographs or on a transparent overlay made 
to the closest 0.1 millimeter, and the micrometer reading on the height finder 
made for the difference in parallax to the precision of 0.01 millimeter, will give 
surprisingly accurate dip results (sec.IV A2). The longer the horizontal distance 
between the upper and lower points in the direction of dip, and the greater 
the difference in height between these two points, the less will be the error in 
the final dip calculations arising from small inaccuracies in the photo measu- 
rements. 
III. Application of theory to trigonometric calculation of dip. 
A. General. 
The two measurements necessary for the trigonometric calculation of dip 
are the adjusted horizontal distance (li us of fig. 1), and the difference in height 
(dh of equation II and fig. 1), as shown in the following equation: 
difference in height 
ten angle dip i diueed-honzonmil distance