C-factor—An empirical evaluation which expresses the vertical (elevation) measuring capability of a stereoscopic system; generally defined as the smallest contour interval which can be plotted to required accuracy.  The C-factor is not a fixed constant. It varies over a considerable range, according to the conditions of the photogrammetric system and the precision of use.  The C-factor is often used to determine the flight height from which aerial photographs should be taken for photogrammetrically accomplishing topographic mapping, at the smallest contour interval accurately plottable from using a particular aerial camera and instrument system. The practicable flight height is the contour interval multiplied by the C-factor. 

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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