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. Traditionally, the reference surface is linked to the mean sea level and the heights are determined using geodetic levelling techniques. These techniques 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. The mean sea level serving as the zero-height level is inferred from averaged tide gauge records over certain time intervals and the heights are determined along the so-called vertical or levelling networks. The equipotential surface that best fits the mean sea level is known as the geoid and it is to be determined from gravity measurements. Thus, the mean sea level determined at a tide gauge to provide the zero-height level for a height system realises a local geoid. Presently, more than 100 local physical height systems are in use.