WGS 84
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 and 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.