Last reviewed 16 Sept 2026 · 8 min read
Electronic distance measurement (EDM)
EDM measures distances by electromagnetic waves instead of tapes. It gives high accuracy over long distances quickly, across rough terrain, rivers and traffic.
Principle — phase comparison
A carrier wave (microwave, infrared or light) is modulated with a measuring wave of known wavelength . The wave travels from the instrument to a reflector and back (double path). The instrument measures the phase difference between transmitted and received waves, which gives the fractional part of a wavelength (); the integer number of wavelengths () is resolved by transmitting several frequencies.
Velocity of electromagnetic waves in air: ( = speed of light in vacuum, = refractive index of air), and .
Pulse (time-of-flight) method — the time taken by a short laser pulse to travel to the target and back gives — used in reflectorless EDM and laser scanners.
Types of EDM
| Type | Carrier | Range and features | Examples (historical) |
|---|---|---|---|
| Microwave instruments | Microwaves (cm wavelengths) | Long range (tens of km); work in fog/haze; need instruments at both ends (master and remote); affected by humidity | Tellurometer |
| Visible light instruments | Modulated visible light | Medium to long range; affected by daylight | Geodimeter |
| Infrared instruments | Infrared (near-IR) with prism reflectors | Short to medium range (a few km); compact; widely used in total stations | Distomat series |
| Laser (reflectorless) instruments | Visible or IR laser | Measure to natural surfaces without a prism over limited ranges; pulse or phase methods | Modern total stations, laser distance meters |
Reflectors
- Corner cube prisms (retro-reflectors) return the beam parallel to its incoming direction — single or multiple prisms for longer ranges; mounted on poles or tribrachs.
- Reflective sheets (targets) for short ranges; reflectorless measurement to walls, rock faces and inaccessible points.
- Each prism has a prism constant (offset) that must be set in the instrument.
Errors and corrections
| Error | Description / correction |
|---|---|
| Zero (additive) error / instrument constant | Offset of the electrical centre from the mechanical centre of the instrument and reflector — determined by calibration on a baseline and applied as a constant |
| Prism constant | Offset due to prism geometry — entered into the instrument |
| Scale error | Due to frequency drift of the oscillator — proportional to distance (ppm); checked by calibration |
| Cyclic error | Periodic error within a wavelength due to electronic interference — calibrated |
| Atmospheric error | Refractive index of air depends on temperature, pressure (and humidity for microwaves) — correction in ppm computed from measured temperature and pressure and entered in the instrument |
| Centring and pointing errors | At instrument and reflector — careful setup, optical/laser plummets |
| Multipath | Reflections from nearby surfaces (especially microwaves) |
Accuracy specification: stated as — a constant part plus a part proportional to distance (e.g. ±(2 mm + 2 ppm)).
Reductions
EDM measures slope distance . With the zenith angle (or vertical angle ):
Further reductions: to mean sea level (or ellipsoid) and to the map projection grid (scale factor) for control surveys.
Total station
A total station is an integrated electronic surveying instrument combining:
- An electronic (digital) theodolite — measures horizontal and vertical angles electronically.
- An EDM — measures slope distances.
- A microprocessor — computes horizontal distances, height differences, coordinates, etc.
- Data storage — internal memory, cards or data collectors, with USB/Bluetooth transfer.
- Display and keyboard (often both faces) with onboard software programs.
- Laser plummet, electronic level (dual-axis tilt compensator), guide lights.
Functions and onboard programs
| Function | Use |
|---|---|
| Angle and distance measurement | Horizontal angle, vertical/zenith angle, slope, horizontal and vertical distances |
| Coordinate measurement | Direct computation of E, N and Z of observed points |
| Stake-out (setting out) | Guides placement of points of known coordinates on the ground — construction layout |
| Resection (free station) | Determining instrument position by observing known points |
| Remote elevation measurement (REM) | Heights of inaccessible points (e.g. power lines, bridge soffits) |
| Missing line measurement (MLM) | Distance and height difference between two observed points |
| Area and volume computation | From observed boundary points |
| Offset measurements, tie distance, traverse, road design programs | Various field tasks |
Setting up and orientation
- Set up the tripod over the station; centre with the laser/optical plummet.
- Level using the circular bubble and electronic level (tilt compensator corrects residual tilt).
- Enter station data — station coordinates, instrument height, prism height, prism constant, atmospheric correction (temperature and pressure).
- Orientation (backsight) — sight a known backsight point (or enter a known azimuth) to set the horizontal circle to the correct bearing.
- Measure detail/control points or stake out.
With horizontal distance , azimuth (whole circle bearing) , vertical difference , instrument height and target (prism) height :
Advanced total stations and related technology
- Robotic (motorised) total stations — automatic target recognition and tracking; one-person operation, machine control.
- Reflectorless total stations — measure to surfaces without prisms.
- Imaging total stations — integrated cameras for documentation and photogrammetric measurement.
- Terrestrial laser scanners — capture dense point clouds (millions of points) for 3D modelling, as-built surveys, heritage documentation.
- Integration with GNSS ("smart stations") and CAD/GIS/BIM software.
Advantages and limitations
| Advantages | Limitations |
|---|---|
| High accuracy and speed | High cost; requires trained personnel |
| Automatic recording — eliminates booking errors | Dependent on batteries and electronics |
| Computations in the field (coordinates, areas, stake-out) | Line of sight required between instrument and target |
| Easy data transfer to computers, CAD and GIS | Atmospheric and prism settings must be correct; errors can be hidden in automated outputs |
| Works on difficult terrain and long distances | Instrument calibration needed periodically |
Applications
Topographic and detail surveys; control traverses; construction layout (buildings, bridges, highways, tunnels); cadastral boundary surveys; deformation monitoring of dams, bridges, slopes and buildings; as-built surveys; volume computations of stockpiles and excavations; mining; accident reconstruction.