(516) 
The absolute difference in height (dH) between U and L (fig. 2) may be 
calculated from the following equation (derived from equation I) in cases where 
the height (H) of the airplane above the lower point of the dip slope or bed- 
ding trace is known: 
(dh) H 
fu idBudp dE 
However, this distance is not needed for the calculations presented in this 
paper. 
(dpH (dpH 
dH = 
D. Measurement of Horizontal Distance. 
Considering the lower point (l) to be on the datum plane, the upper point 
(u) has been displaced in relation to |, depending upon the difference in height 
(dh) between these two points. It is possible to adjust for this displacement by 
measuring the total net radial displacement at u by the methods described in 
section IIIB and D. The corrected horizontal distance obtained is the distance 
at right angles to the strike from | to the adjusted position of u. The relation- 
ship of this corrected horizontal distance to the photo positions of u and I is 
shown by line us l; on figure 1. 
E. Possible Sources of Error in Photogrammetric Dip Calculations. 
1° Tilt. 
Tilt is potentially the largest source of error in dip calculations. In com- 
mercial aerial photography, the absolute tilt on any single photograph is gener- 
ally less than 1° and rarely as much as 3°, and the relative tilt between two 
adjacent photographs is rarely as much as 5°. Many variables enter into the 
problem of the effect by tilt on the dip calculations. The direction of tilt of 
the two photographs of the stereo pair relative to each other is significant. 
That is, if the two directions of tilt parallel each other the effect will be less 
than if the directions are not parallel, having a maximum effect when the direc- 
tions of tilt of the two photographs are in opposite directions. Nevertheless, a 
small amount of tilt may considerably affect the dip calculations, especially as 
the direction of tilt more closely approaches the direction of the line of flight, 
and as the direction of dip more closely approaches the direction of tilt. Both 
relative and absolute tilt can affect the calculated dip. These relationships have 
been analyzed graphically, and are discussed in fuller detail in section IV A6. 
Tilt can be corrected as mentioned in section IIA. However, this involved 
correction is seldom made because the average tilt will allow an approximately 
correct dip calculation to be made. 
2. Difference in Altitude of the Airplane at Two Adjoining Air Stations. 
Generally the difference in altitude of the airplane at the two air stations is 
small in relation to the height of the airplane above the ground, and has little 
effect upon the dip solutions. However, this factor can be corrected at the 
same time the correction for tilt is made. 
3. Unadjusted Photo Base. 
The net effect in the final dip result of adjusting the measured photo base 
to that of the elevation of the lower (l) or upper (u) point is small (generally 
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