 A method of establishing precise geodetic baselines , invented by Yrjö Väisälä , exploited the low coherence length of white light . Initially , white light was split in two , with the reference beam " folded " , bouncing back @-@ and @-@ forth six times between a mirror pair spaced precisely 1 m apart . Only if the test path was precisely 6 times the reference path would fringes be seen . Repeated applications of this procedure allowed precise measurement of distances up to 864 meters . Baselines thus established were used to calibrate geodetic distance measurement equipment , leading to a metrologically traceable scale for geodetic networks measured by these instruments . ( This method has been superseded by GPS . ) 
