208 
IOWA ACADEMY OF SCIENCE Voi,. XXVII, 1920 
In figure 41 let iC be the plate in the tilted camera. Let P be 
the image of a point on the path of the loop, the ray coming 
from the direction S, through the center of the lens C. The focal 
length f is given by the distance CO. Take the horizontal line 
OX and the line OY perpendicular to it as axes of reference. 
CRN is a vertical line through C. OF is in the vertical plane 
through OC. Horizontal lines OR and MN in this vertical plane 
intersect CRN in R and N respectively. The angle MNP, called 
c, is the angle which a vertical plane through SCP makes with 
the vertical plane through the axis OC. This angle is the azimuth 
of the ray SP referred to the horizontal direction MN. The angle 
CPN, called a, is the elevation of the ray SCP above the hori- 
zontal plane. Letting CP=r, then 
^2 == ^2 -j- y2 -}- J^2 ( 1 ) 
CN = CR + RN = f sin e y cos e, 
where e is the elevation of the axis of the camera above the 
horizontal plane. Then 
y cos ^ / sin e 
sin a = (2) 
and since 
PN = r cos a 
We thus have formulas for determining, as viewed from one 
camera the elevation of a given point on the path of the airplane 
above the horizontal plane, and its azimuth with reference to a 
fixed direction from the base line. A great deal is here omitted 
in regard to the problem of choice of camera, plates, camera 
adjustment, lens distortion, determination of focal length, de- 
termination of the azimuth of the camera axis, etc. One also 
