Handbook of Civil Engineering Calculations

photograph to the corresponding distance along the ground. In this case, the ratio is 1
cm/120 m = 0.01 m/120 m = 1/12,000.
Let H = flying height; h = elevation of A above datum;/= focal length; S = scale of
photograph, expressed as a fraction. From Fig. 29, S = oalOA, and by similar triangles S =
fl(H-h).

(a) Elevation normal to vertical plane through

optical axis and point A

FIGURE 29

Datum plane

Ground

Plane of

photograph

Vertical plane

through optical

axis and A

Positive photograph

(b) Plan

Opticalaxis