(522) 
D. Alternate Procedures to Measure the Corrected Horizontal Distance. 
1. Graphic Method. 
This method is based on the fact that a line drawn under the stereoscope 
in the direction of dip will not be at right angles to the line of strike plotted 
on a single photograph (fig. 3), except in the special case of the stereoscopic dip 
line passing through the photo center. In contrast, a strike line drawn under 
the stereoscope will have the same direction as the strike line drawn on a single 
photograph. 
Two cases are considered: the first in which u and | are in the direction 
of dip on the stereo model, and the second in which they are not. The first 
case needs but three constrution lines and one photo measurement. The detail- 
ed procedure to be used in the first case is as follows: 
a. A transparent acetate strip on which a fine ink line is ruled, is placed 
on the photograph so that the line passes through the photo center and the 
upper point, and fastened in place with scotch drafting tape. 
b. Another similar strip is placed on the same photograph, oriented so 
that the line passes through the lower point in the direction of strike, and is 
also fastened in place. 
c. A measurement is made on the photograph at right angles to this strike 
line from the lower point to the radial line passing through the upper point. 
This distance (lus of fig. 3) is the adjusted horizotal distance between u and |, 
and should be measured to the closest 0.1 millimeter. 
In the second case, where | and u are not on the line of dip as seen under 
the stereoscope, the procedure is slightly different. 
a. An acetate strip similar to that used above is placed on the photo- 
graph so that the line passes through u in the direction of strike, and is fastened 
in place with scotch drafting tape. 
b. Another similar strip of acetate is so placed that the line passes through 
| in the direction of dip (seen under the stereoscope), intersectiing the strike 
line. 
c. The intersection of these two lines will be equivalent to the position of 
u in the first case described above. The method already described above can 
now be used for finding the corrected horizontal distance. 
In this graphic method, the distance between 1 and us can be scaled more 
accurately in those cases where the radial line through u approaches the direc- 
tion of strike. The inaccuracy becomes potentially greater as the radial line 
through u approaches the direction of dip (i.e., at right angles to the strike). 
This is so because the point of intersection of the radial line ciu and the line 
through 1 at right angles to the strike (fig. 3) can be located more precisely as 
their angle of intersection approaches 90 degrees. If the radial line through u is 
in the direction of dip under the stereoscope, the corrected horizontal distance 
cannot be solved for graphically on that photograph, because the two lines will 
be coincident and there will be no intersection. Therefore, the one photograph 
of the stereo pair which will give the more accurate measurement as just des- 
cribed must be chosen. 
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