Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual - Manual (Page 129)

119
Mapping Plane Scale
Factor
When converting distances on a map to distances on the ground, you
must correct for two different scale distortions. First, the effects of the
map projection distortion must be corrected with the mapping plane
scale factor. Second, the geometric effect of your height above the
reference surface (ellipsoid height) must be corrected with the
ellipsoid scale factor. Generally, these two scale factors are multiplied
together into the combined scale factor. Although the scale factor is
computed with differential equations of the map projection, one can
visualize it in a geometric sense. Consider the following diagrams: