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). 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. 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. As the tide gauges register the local sea level and the vertical networks cover limited regions, these systems are known as local height systems. Presently, there are about hundred local and regional height systems in use, and they exhibit discrepancies with respect to each other up to \pm2 m.