WGS84
Efforts to supplement the various national surveying systems, which begun in the 19th century with the foundation of the "Mitteleuropäische Gradmessung", resulted in a series of global ellipsoids of the Earth (e.g., Helmert 1906, Hayford 1910/1924) which would later lead to develop the World Geodetic System. Nowadays the practical realisation of the metre is possible everywhere thanks to the atomic clocks embedded in GPS satellites.