What is Electronic Distance Measurement?
Electronic distance measurement (EDM) (also called laser ranging) uses lasers and mirrors to measure the mountain’s movement. A theodolite emits a laser pulse to a mirror prism that’s installed on the moving rock face. The instrument measures how long it takes the laser beam to travel from the theodolite. In theory, as the rock face moves down the mountain slope (toward the survey instrument), the distance between the instrument and the prism will decrease. Using this time, we can precisely measured the distance to one part per million (i.e., one millimetre over a distance of one kilometre).
