10 
4. Definitions of Interior Orientation 
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In the first approximation the picture is assumed to be a mathematical cent 
ral projection of the object space on the image plane. All rays between the 
points in the object and corresponding points in the image are straight lines 
which intersect in one point, the center of projection. Fig. 2. The elements of 
interior orientation now have to give the position of this center in relation to 
the image. As a rule fiducial marks are introduced in the picture to serve as a 
reference. Theoretically one point and one direction is sufficient, but in prac 
tice two or more fiducial marks are used. Having the fiducial reference, three 
quantities are necessary to locate the center of projection. These quantities can 
IMAGE PLANE 
Fig. 2. 
Central projection. 
Straight lines passing 
through the projection cen 
tre connect the object 
points and the image 
points. The image is assu 
med to be a plane. The 
classic definition of in 
terior orientation is given 
by the principal point and 
the camera constant.