Reducing the measured geometric quantities such as distances, angles, azimuths, and heights from the surface of the Earth to the reference ellipsoid, to geodesics, geodetic azimuths, and height, is necessary for using ellipsoidal geometry in computing the coordinates of points, positioning, or mapping. After performing these computations, the geometric quantities on the surface of the ellipsoid can be transferred back to the Earth's surface. This process is crucial and practical for boundary determination and the setting out of large projects such as railways and roads.
This short video lecture explains this principle in a straightforward manner.
