would be measured and 
he photo center of the 
enter of the left-hand 
ljusted photo base (bu) 
o the factor (bi + dp). 
e photo center marked 
rked from that photo- 
meters from the upper 
from the second point 
ph to the other upper 
n of these two lines 
rom the lower point 
rrected position of the 
: is the corrected hori- 
calculated, as shown in 
arallax, adjusted photo 
\ctual calculations may 
| Trace. 
eedle point, welldefined 
ight-hand photograph. 
' direction of strike as 
r in direction of dip as 
, place a small strip of 
n which a fine ink line 
asses through the lower 
photograph by scotch 
h, oriented so that the 
red center; if the right- 
nts coincide; if the left- 
oriented as in step 1, as 
echt angles from the line 
tion of the upper point. 
o above. 
ice in Height (db). 
ce in height (dh) between 
3ethod of measuring the 
(521) 
difference in parallax and the adjusted photo base, although the same equation 
III is used for the final calculation. 
a. Height Finder Used to Measure the Difference in Parallax. The Abrams 
height finder is the only instrument described in this paper for measuring in- 
strumentally the difference in parallax, because it is simple, small, accurate, uses 
the magnifying lens-type stereoscope, and is generally available. 
For any method of instrumental measurement of difference in parallax, 
the two photographs of the stereo pair must be oriented as precisely as possible 
in relation to the line of flight. This is done as described in the following steps: 
i. Place a thin strip of clear acetate about 1.5 cm wide on which a thin 
ink line has been ruled across the right-hand photograph so that the line passes 
through the photo center and the transferred center marked from the left- 
hand photograph. This strip should extend completely across the photograph, 
and in most cases, will not pass through the other transferred center. Fix in 
place on the photograph by scotch drafting tape. (The acetate strip has the 
advantage of not marking the photograph, of bearing a line that is always thin 
and straight and of being easily adjusted. 
ii. Place a similar strip on the left-hand photograph oriented in a similar 
manner, and fix in place. 
ii. Place one of the photographs of the stereo pair on the desk top so 
that the ruled line of the acetate strip will line up with a fine line already ruled 
across the desk top. 
iv. Place the other photograph in approximately the correct position for 
use with the pocket stereoscope so that the ruled line of the acetate strip will 
line up with the ruled lines of both the first photograph and the desk top. 
v. Using the stereoscope, shift one photograph until stereoscopic vision 
is obtained, keeping the ruled lines in position. 
After the stereo pair is properly oriented as described above, the floating 
dots of the height finder are made to fuse first at the lower point, and then at 
the upper point. The difference in micrometer readings will be the difference 
in parallax (dp) between these points. The average difference of several sets of 
micrometer readings (four or five usually are necessary and sufficient) made to 
the closest 0.01 millimeter should be used as the average difference in parallax 
in the equations of this paper. 
b. Height Finder Used to Obtain Adjusted Photo Base. In this method, 
the height finder is used to measure the difference in parallax between the 
transferred center and the lower point. This difference in parallax should be 
added to the measured photo base in cases where the elevation of the trans- 
ferred center 1s lower than the lower point; where higher, the difference in paral- 
lax should be subtracted. This adjusted photo base (bi) is based on the elevation 
of the lower point. To this figure should be added the difference in parallax 
between the upper and lower points to obtain the factor (b: + dp). 
However, to adjust the measured photo base directly to the datum plane 
passing through the upper point, the difference in parallax between the trans- 
ferred center point and the upper point should be measured. The measured 
photo base should be adjusted by the amount of this difference in parallax, as 
described above, to obtain the adjusted photo base (bu) based on the elevation 
of the upper point, and which is equivalent to the factor (bi + dp). 
40