how is a metre defined
The metre is defined as the path length travelled by light in a given time and practical laboratory length measurements in metres are determined by counting the number of wavelengths of laser light of one of the standard types that fit into the length, and converting the selected unit of wavelength to metres.n 1959, the imperial inch was re-defined as 0.0254 metres (2.54 centimetres or 25.4 millimetres). One metre is about 3 3â8 inches longer than a yard, i.e. about 39 3â8 inches.