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.