Horizontal Distance. 
vn under the stereoscope 
he line of strike plotted 
se of the stereoscopic dip 
strike line drawn under 
ce line drawn on a single 
nd | are in the direction 
they are not. The first 
neasurement. The detail- 
Ik line is ruled, is placed 
1e photo center and the 
g tape. 
photograph, oriented so 
irection of strike, and is 
-ight angles to this strike 
hrough the upper point. 
istance between u and |, 
line of dip as seen under 
is placed on the photo- 
of strike, and is fastened 
it the line passes through 
, intersectiing the strike 
ivalent to the position of 
idy described above can 
ince. 
id u4 can be scaled more 
| u approaches the direc- 
reater as the radial line 
ht angles to the strike). 
dial line ciu and the line 
ocated more precisely as 
e radial line through u is 
ected horizontal distance 
'ecause the two lines will 
ore, the one photograph 
measurement as just des- 
(523) 
2. Radial Displacement Method. 
This method which has been described by L. J. Desjardins!) combines a 
graphic and a caluculated solution to obtain the corrected horizontal distance 
(lus) between the upper and lower points. The method as originally described 
consists of calculating the total net radial displacement at the lower point due 
to difference in elevation between the upper and lower points. As modified to 
fit the geometry developed in this paper, all the displacement is considered to 
be at the upper point, and the following equation is used (see fig. 2): 
C1 U1 (dp) C1 u1 (dp) 
MR m 
where du is the radial displacement of the upper point due to difference in 
elevation between the upper and lower point 
ciui is the photo distance from the photo center to the upper point 
dp is the difference in parallax between the upper and lower points 
bi is the adjusted photo base, based on the elevation of the lower point 
bu is the adjusted photo base, based on the elevation of the upper point. 
The distance (du) is scaled on the photograph radially toward the photo 
center from the upper point, giving the corrected horizontal position of the 
upper point. If the upper and lower points are on the line of dip from each 
other, the distance from the lower point to the corrected position of the upper 
point will be the corrected horizontal distance. If the two points are not on 
the line of dip, the lower point must be projected along the line of strike by a 
ruled transparent strip. A measurement normal to this line, from the line to 
the corrected position of the upper point, will give the corrected horizontal 
distance between the two points. 
IV. Conclusions. 
A. Accuracy Possible. 
1. General. 
Although there are many variables in the calculations for the dip measure- 
ments, the general range in accuracy of calculated dips is within plus or minus 
10 percent of the observed field dips. This figure has been obtained after two 
field problems were worked, and although the data are scant, it is indicative. To 
a large extent the accuracy depends upon the precision of the individual meas- 
urements, which in turn depends upon the magnitude of these measurements. 
Measurements of dips approaching horizontal or vertical may be generally less 
accurate than those dips in a middle range. This is so because dips close to hori- 
zontal generally have small differences in parallax, and dips close to the verti- 
cal have short corrected horizontal distances. 
A higher degree of accuracy may be expected from a worker after he has 
become familiar with photogrammetric measurements. Specifically, the stere- 
oscopic locating and transfer of points, and measurements made with the height 
finder become more accurate. 
Furthermore, the accuracy of this photogrammetric method for calcu- 
lating the angle of dip depends in part upon the sharpsightedness of the oper- 
ator. That is, the smaller the difference in angular convergence of his lines of 
1) Desjardins, L. J., Techniques in photogeology: Am. Assoc. Petroleum Geologists Bull, 
vol. 34, no. 12, pp. 2284—2317, 1950.