(524) 
sight that he can detect, the more accurately can he operate the height finder 
or transfer points under the stereoscope. 
The quantitative differences quoted below in section IV A2 are based on 
a focal length of 8!/4 inches (209.6 millimeters), but are of the same general 
magnitude for other focal lengths. 
2. Effect of Measurements on Accuracy of Dip Calculations. 
a. The Adjusted Photo Base. This is the longest measurement to be made 
on the photographs, and need only be measured to the closest 1.0 millimeter. 
A difference of 1.0 millimeter in the measurement of the photo base will affect 
the final dip result by not more than half a degree in all cases except possibly 
where the measurements of the difference in parallax, adjusted horizontal 
distance, and the adjusted photo base are extremely large or small. 
However, because the adjusted photo base can be read easily to 0.1 milli- 
meter on the transparent overlay, it is suggested this degree of precision be 
practiced even though it is not absolutely necessary. 
b. The Corrected Horizontal Distance. This distance on the transparent 
overlay should be made to the closest 0.1 millmeter. The longer this distance, 
the less will be the difference in the dip result caused by the difference of 0.1 
millimeter in reading the measurement. A difference of 0.1 millimeter in the 
range of approximately 5 millimeters will affect the final dip result by less than 
half a degree. However, in the range near 1.0 millimeter this same difference 
may affect the final dip result by as much as 2'/» degrees, with increasing error 
in dip result as the adjusted horizontal distance becomes smaller. 
As indicated by one field check, if the adjusted horizontal distance is less 
than 0.7 millimeter, the final dip result will be generally less accurate than dips 
calculated with a larger horizontal distance. This figure of 0.7 millimeter 1s an 
arbitrary figure based on meager data, but it gives an idea if the general degree 
of magnitude of the lower limit of the corrected horizontal distance consistent 
with acceptable accuracy. 
c. The Difference in Parallax. This distance should be measured from 
the transparent overlay to the closest 0.1 millimeter. Because in most cases, this 
is the shortest distance measured from the photographs, and error of 0.1 milli- 
meter will have a greater effect upon the final dip result than the same error 
in other measurements. In one specific case solved for a given dip of 35 degrees, 
an error in measuring of 0.1 millimeter at the range of 2.0 millimeters affected 
the final calculated dip by 1!/» degrees, whereas the same error in the range of 
0.2 millimeter affected the dip by 9'/» degrees. The greater the vertical distance 
between the upper and lower points, with the consequent larger difference in 
parallax, the less the final dip result will be affected by small errors in mea- 
suring the difference in parallax. 
3. "Transfer of Points. 
The upper and lower points must be transferred carefully from one photo- 
graph to the other, and then to the transparency. Any difference will be 
directly reflected in the measurement of the difference in parallax. 
To check the accuracy of the stereoscopic transfer of points, a small inked 
circle of the same size can be spun around each of the two points. Under the 
stereoscope, these circles will appear to fuse and to float horizontally at the 
elevation of the point if the point has been trasferred carefully. 
  
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