Fig. 1. The principles of the y-parallax method for the determination of radial 
distortion. 
are zero. Due to the radial distortion of the pictures there will remain 
residual y-parallaxes in many other points of the model. The size and 
direction of these residual y-parallaxes due only to the radial distortion 
can be determined with elementary procedures from a known distortion 
curve. Such procedures have frequently been demonstrated in the pho- 
togrammetric literature. See for instance [1]. 
We will now, however, instead determine the distortion curve from 
measured y-parallaxes. This can be performed with a very simple pro- 
cedure, if the y-parallaxes are measured in certain points of the model. 
See fig. 1. The normal orientation points for the relative orientation are 
denoted 1—6. Also nine points (1—9) can be used. 
We draw a circle around point 1 through 3 and 5, and another circle 
around point 2 through 4 and 6. The radii of these two fundamental 
circles are d. We denote parts of the circles (20—3), (10—4), (20—5), 
(10—6) by 3, 4, 5 and 6 respectively (see fig. 1). Then we draw a se-