Structure from Motion find camera poses & a structure to an unknown scale of the real metric measure. In order to restore the correct scale and orientation to known 3D coordinates, OpenMVG provide a GUI to perform Ground Control Point registration.
Given some 3D known ground control points and their 2D observations in some images, we can compute the rigid transformation to move the SfM scene to the known 3D coordinates. The rigid transformation (7Dof) is computed in least square between the triangulation of the observations and the known 3D positions).
For your ground control points your can use specific pattern or object for which you known the measures (post-it, ruller, calibration target…).
Figure : Ground Control Point registration principle illustration.¶