perate the height finder 
ion IV A2 are based on 
are of the same general 
ulations. 
measurement to be made 
ie closest 1.0 millimeter. 
he photo base will affect 
all cases except possibly 
lax, adjusted horizontal 
ze or small. 
read easily to 0.1 milli- 
is degree of precision be 
ance on the transparent 
[he longer this distance, 
by the difference of 0.1 
of 0.1 millimeter in the 
al dip result by less than 
ter this same difference 
es, with increasing error 
ies smaller. 
orizontal distance is less 
y less accurate than dips 
e of 0.7 millimeter is an 
dea if the general degree 
ontal distance consistent 
ould be measured from 
ecause in most cases, this 
s, and error of 0.1 milli- 
ult than the same error 
given dip of 35 degrees, 
2.0 millimeters affected 
me error in the range of 
ater the vertical distance 
uent larger difference in 
by small errors in mea- 
arefully from one photo- 
Any difference will be 
e in parallax. 
of points, a small inked 
: two points. Under the 
loat horizontally at the 
carefully. 
(525) 
4. Strike or Dip Projections. 
The longer a construction projection of either strike or dip, the less accu- 
rate may be the dip result. This was demonstrated on one traverse where sev- 
eral cases involving a projection of strike through the lower point of more than 
5 millimeters showed less average accuracy than for cases involving shorter 
strike projections. 
5. Scale of the Photograph. 
The average scale of the photograph is significant insofar as larger scales 
will show a longer photo distance for any specific ground distance. The im- 
portance of long photo distances has been discussed above. Photographs having 
a scale of 1: 20,000 or larger consequently are preferred over those having a 
scale of 1 : 30,000 or 1 : 40,000. In addition, with larger scales, more geologic 
interpretation can be made because of the greater detail apparent on the photo- 
graphs. However, modifying the importance of the scale of the photograph 
for the amount of interpretation possible is the apparent difference of relief on 
the three-dimensional stereoscopic image seen under the stereoscope. This is 
governed by the angular field of the camera and the altitude of the airplane 
above the ground. Assuming the standard 60 percent overlap of two adjacent 
photographs along the line of flight, the minimum angular field of the camera 
should be about 40 degrees for photogeologic interpretation. 
6. Til. 
Tilt may affect dip calculations appreciably. The calculated dip is affected 
by the following significant factors — the direction of tilt relative to the line 
of flight, the direction of tilt of the two photographs of the stereo pair relative 
to each other, the direction of dip relative to the direction of tilt, and the 
amount of tilt. Lesser factors are the focal length, the adjusted photo base, the 
difference in height between the upper and lower points, and the distance from 
these points to the photo centers of the stereo pair. 
Either or both photographs of the stereo pair may be tilted. For a graphic 
analysis of the effect of tilt on the dip result, only relative tilt was considered; 
that is, one photograph remaining truly vertical, and the other photograph 
being tilted. In several test cases solved graphically, the following observations 
were made. Tilt in the direction of the line of flight has more effect upon the 
dip calculations than any other of the above single factors. Conversely, tilt at 
right angles to this direction generally has no appreciable effect. The adjusted 
photo base is the measurement generally most affected whereas the difference 
in parallax and the adjusted horizontal distance are affected but slightly. In 
addition, tilt will have more effect if the direction of dip parallels the direction 
of tilt rather than at right angles to it. In the graphic cases solved having a tilt 
of 3° in the direction of the line of flight, with the direction of dip paralleling 
this, in the range of dip from 20° to 60°, a maximum difference of dip result of 
4? was noted (based on a focal length of 8'/, inches and an adjusted photo base 
of 82 millimeters). 
The presence of relative tilt is suggested by the two following conditions: 
1) the divergence of the position of one photo center from the usual uniform 
spacing of the photo centers along a nearly straight line of flight, and 2) the 
difference in the distance normal to the line of flight from this line to the same 
point on the two photographs of the stereo pair.