7. Tangential Distortion 
  
The tangential distortion is positive when it appears as a counter 
clockwise displacement on the focal plane to an observer looking from 
the focal plane towards the lens. The datum from which the displacement 
is measured is a line on the focal plane passing through the fiducial 
centre which gives symmetrical values of tangential distortion for image 
points on either side of the fiducial centre. This defect does not 
occur in a perfectly centred and symmetrical lens. 
8. Standard Distortion 
The radial distortion of a given lens type, for which subsequent 
elements in the plotting system have been designed, and to which indivi- 
dual lenses are reouired to conform. The standard distortion may or may 
not be zero. 
9. Calibration Principal Distance 
The value of the principal distance chosen in calculating the 
distortion values ouoted in the calibration certificate. It is chosen 
to bring the measured radial distortion as close as possible to the 
standard distortion values throughout the field. 
10. Centre of Symmetry 
In an asymmetrical or badly centred lens, the point about which 
radial distortions are most nearly symmetrical. 
Lt, Principal point of Auto-collimation 
  
The image formed on the focal plane by the camera lens from an 
incident beam of parallel light which in the object space is perpendicu- 
ler to the emulsion. 
12. Reseau Datum Cross 
The centre cross along the right hand edge of the register glass 
when looking at the register glass towards the lens, with the serial 
number of the reseau located along the bottom edge of the glass. 
IS. Reseau Errors 
The displacements from the nominal position of each reseau cross 
in a co-ordinate system whose origin is the fiducial centre, whose X 
direction is parallel to the line joining the fiducial centre to the 
reseau datum cross, end whose Y direction is at right ancles to the 
X direction. 
A positive error in the X direction indicates that the cross is 
nearer the reseau datum cross than its nominal position. 
A positive error in the Y direction indicates that the cross is 
nearer the edge containing the reseau serial number than its nominal 
position. 
  
  
  
  
  
  
  
  
  
  
  
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