Calculation Procedure:
- Establish the relationship between elevation and parallax
Two successive photographs are said to overlap if a certain amount of terrain appears in
both. The ratio of the area that is common to the two photographs to the total area appear-
ing in one photograph is called the overlap. (In practice, this value is usually about 60
percent.) The distance between two successive exposure stations is termed the air base. If
a point on the ground appears in both photographs, its image undergoes a displacement
from the first photograph to the second, and this displacement is known as the parallax of
the point. This quantity is evaluated by using the micrometer of a parallax bar and then
increasing or decreasing the micrometer reading by some constant.
Assume that there is no change in the direction of flight. As stated, the x axis in the
photograph is parallel to the line of flight, with x values increasing in the direction of
flight. Refer to Fig. 32, where photographs 1 and 2 are two successive photographs and
the subscripts correspond to the photograph numbers. Let A denote a point in the overlap-
ping terrain, and let a denote its image, with the proper subscript. Figure 32c discloses
thatjla =y 2 ai thus, parallax occurs solely in the direction of flight. Let/? = parallax and B
= air base. Then/? = xla — x2a = O 1 W 1 — O 2 Jn 2 = O 1 W 1 + m 2 o 2. Thus, W 1 W 2 = B —p. By pro-
portion, (B -p)IE = (H-h ~f)/(H- h\ giving piB =fl(H- h\ or/? = BfI(H- K) 9 Eq. a.
Thus, the parallax of a point is inversely proportional to the vertical projection of its dis-
tance from the front nodal point of the lens.
- Determine the flying height
From the given data, B = 768 m and/= 152.6 mm. Take sea level as datum. For the con-
trol point, h = 284 m and/? = 11.37 + 76.54 = 87.91 mm. From Eq. a,H=h+Bf/p, or H
= 284 + 768(0.1526)70.08791 = 1617 m.
- Compute the elevation of P
For this point, p = 15.41 + 76.54 = 91.95 mm. From Eq. a, h = H - BfIp, or h = 1617 -
768(0.1526)70.09195 = 342 m above sea level.
DETERMINING AIR BASE OF OVERLAPPING
VERTICAL PHOTOGRAPHS BY USE OF
TWO CONTROL POINTS
The air base of two successive vertical photographs is to be found by using two control
points, R and S, that lie in the overlapping area. The images of R and S are r and s, respec-
tively. The following data were all obtained by measurement: The length of the straight
line RS is 2073 m. The parallax of R is 92.03 mm, and that of S is 91.85 mm. The coordi-
nates of the images in the left photograph are xr = +86.46 mm, yr = -54.32 mm, xs =
+29.41 mm, andys = +56.93 mm. Compute the air base.
Calculation Procedure:
- Express the ground coordinates of the endpoints in terms
of the air base
Refer to Fig. 32, and letXand /denote coordinate axes that lie vertically below the Jc 1 and
yi axes, respectively, and at the same elevation as A. Thus, O 1 is the origin of this system
of coordinates. With reference to point ^4, by proportion, XA/xla = YAlyla = (H- K)If. From
the previous calculation procedure, (H — K)If = BIp. Omitting the subscript 1, we have
