Britannica Sources used in this answer

How does electronic distance measurement calculate the distance between survey stations?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

surveying Figure 1: Photogrammetric photographs from two short, overlapping flight strips arranged for supplying mapping details. Photo-control points are shown on only one photograph; shading indicates a typical terrain feature such as a lake (see text).

In civil engineering, electronic distance measurement (EDM) determines the distance between survey stations by electronically timing the passage of electromagnetic radiation—such as light, infrared radiation, or ultrahigh-frequency radio beams—as it travels to a target and back.1

The process functions as follows:

Signal Transmission: A transmitter-receiver at the starting station sends a beam toward a remote station. For light-based systems, the remote station uses corner mirrors to reflect the beam; for radio-based systems, a retransmitter is required.1
Phase Shift Detection: The distance is calculated based on the time taken for the beam to complete the round trip. This elapsed time is determined by measuring the shift in phase of a modulating signal superimposed on the carrier beam.1
Conversion: Electronic circuitry detects this phase shift and converts it into units of time, which corresponds to the distance.1
Accuracy: Using multiple modulating frequencies allows the device to eliminate ambiguities that might occur with a single frequency.1

This technology enables trilateration, allowing surveyors to measure distances as accurately and easily as angles.1