A height system is a one-dimensional coordinate system used to express the metric distance (height) of a point above a reference surface (i.e., the zero-height level) along a well-defined path. Particularly, the physical heights are determined using geodetic levelling techniques that measure the distance between two equipotential or level surfaces of the Earth’s gravity field and provide the height along the curved plumb line. As the height determination depends on the level surfaces and the plumb line of Earth’s gravity field, these coordinate systems are called physical height systems. These coordinate systems are called physical height systems as the height determination depends on the level surfaces and the plumb line of Earth’s gravity field. The physical heights are referred to the geoid, a particular equipotential surface of the Earth’s gravity field that is close to the mean sea level. From the practical point of view, the mean sea level has been assumed as the zero-height level and inferred from averaged tide gauge records over certain time intervals. Since the mean sea level differs from the geoid (up to 2 m at global scale) and from point to point, the heights determined along the so-called vertical or levelling networks referred to a given tide gauge define a local height system. Presently, more than 100 local physical height systems are in use.