Siemens ARCADIS Orbic Quick Manual page 58

Postprocessing
(1)
(4)
(5)
58
Distances cannot be measured directly in
the images, since the position of the object
in the beam path and thus the magnification
effect is not clear. A calibration must be
performed first before a distance can be
clearly determined.
(2)
(3)
The calibration object must be perpendicular
to the central ray in the area of the structure
to be measured (generally in the center of
the measuring field to avoid I.I. distortion).