The Si unit of length, the metre, was taken to be the distance between two lines marked on the international prototype bar of platinum-iridium alloy kept at the International Bureau of Weights and Measures (BIPM), Sevres, France. This definition of the metre was replaced in 1960 by a more precise one based on the wavelength of certain light and, subsequently, by one based on the speed of light.

Two Methods are used to measure lengths beyond 10² m/What is the SI unit for volume? m: (i) Triangulation and parallax methods involving angular measurements (ii) reflection methods using radio waves, ultrasound and laser. Both triangulation and reflection methods are used to measure large distances and heights on the Earth.