• Electronic distance measuring instrument is a surveying instrument for measuring distance electronically between two points through electromagnetic waves.
  • Electronic distance measurement (EDM) is a method of determining the length between two points, using phase changes, that occur as electromagnetic energy waves travel from one end of the line to the other end.
  • As a background, there are three methods of measuring distance between two points:

1. DDM or Direct distance measurement

This is mainly done by chaining or taping.

2. ODM or Optical distance measurement

This measurement is conducted by tacheometry, horizontal subtense method or telemetric method. These are carried out with the help of optical wedge attachments.

3. EDM or Electromagnetic distance measurement : –

The method of direct distance measurement cannot be implemented in difficult terrains. When a large amount of inconsistency in the terrain or large obstructions exist, this method is avoided.


  • As an alternative to this optical distance measurement method was developed. 
  • Still, it gained a disadvantage of a limited range of measurement.
  • It is limited to 15 to 150m with an accuracy of 1 in 1000 to 1 in 10000.
  • Above all we have EDM with an accuracy of 1 in 105, having a distance range of 100km.
  • Electronic distance measurement, in general, is a term used as a method for distance measurement by electronic means.
  • In this method, instruments are used to measure the distance that relies on propagation, reflection and reception of electromagnetic waves like radio, visible light or infrared waves.
  • Sunlight or artificially generated electromagnetic wave consists of waves of different lengths.
  • Among these waves microwaves, infrared waves and visible light waves are useful for the distance measurement.
  •  In EDM instruments these waves are generated, modulated and then propagated. They are reflected at the point up to which distance is to be measured from the instrument station and again received by the instrument.
  • The time taken by the wave to travel this 2x distance may be measured and knowing the velocity of the wave, the distance may be calculated. 
  • However, time is too short, measuring the time taken is difficult. 
  • The improved techniques use the phase difference method in which the number of completed wave and an incomplete wave is measured. Knowing the length of the wave, distances are calculated.
  • Built up microprocessors provided in the instrument calculate the distances and display it by liquid crystal display (LCD).

Types of Electronic Distance Measurement Instrument: –

EDM instruments are classified based on the type of carrier wave as: –

  1. Microwave instruments
  2. Infrared wave instruments
  3. Lightwave instruments.

1. Microwave Instruments: –

These instruments make use of microwaves. Such instruments were invented as early as 1950 in South Africa by Dr T.L. Wadley and named them as Tellurometers. The instrument needs only 12 to 24 V batteries. Hence they are light and highly portable. Tellurometers can be used in the day as well as in the night.

The range of these instruments is up to 100 km. It consists of two identical units. One unit is used as a master unit and the other as a remote unit. Just by pressing a button, a master unit can be converted into a remote unit and a remote unit into a master unit. It needs two skilled persons to operate. A speech facility is provided to each operator to interact during measurements.

2. Infrared Wave Instruments: –

  • In this instrument, amplitude modulated infrared waves are used. Prism reflectors are used at the end of the line to be measured. 
  • These instruments are light and economical and can be mounted on theodolite. With these instruments, the accuracy achieved is ± 10mm.
  • The range of these instruments is up to 3 km.
  • These instruments are useful for most of the civil engineering works. These instruments are available in the trade names DISTOMAT DI 1000 and DISTOMAT DI 55.

3. Visible Light Wave Instrument: –

  • These instruments rely on the propagation of modulated light waves. This type of instrument was first developed in Sweden and was named as Geodimeter.
  • During night its range is up to 2.5 km while in the day its range is up to 3 km. The accuracy of these instruments varies from 0.5 mm to 5 mm/km distance.
  • These instruments are also very useful for civil engineering projects.

Error in Electronic Distance Measurement Instruments: –

1. Personal Errors

  • Inaccuracy in initial setups of EDMs and the reflectors over the preferred stations.
  • Instrument and reflector measurements going wrong.
  • Atmospheric pressures and temperature determination errors.

2. Instrumental Errors

  • Calibration errors.
  • Chances of getting maladjusted time to time generating frequent errors.
  • Errors are by the reflectors.

3. Natural Errors

  • Atmospheric variations in temperature, pressure as well as humidity. Microwave EDM instruments are more susceptible to these.
  • Multiple refractions of the signals.

The advantage of using EDM instruments is the speed and accuracy in measurement. Several obstacles to chaining are automatically overcome when these instruments are used.


Origin of Electronic Distance Measurement

  • Electronic distance measurement can be done by instruments like geodimeter, tellurometer or distomat etc. 
  • The first EDM instrument called geodimeter was developed in Sweden in the year 1948. Geodimeter is geodetic distance meter developed based on a modulated light beam.
  • The second instrument for EDM was designed and developed in Africa in the year 1957, named tellurometer.
  •  This instrument employs modulated microwaves.
  • As years passed technology has improved drastically. 
  • At present, we have modern EDMs that displays distance in digital form and many gains microcomputers that calculates horizontal and vertical distance i.e. DX and DY. 
  • They also show sloped distance (DH).
  • Electronic distance measurement equipment is incorporated along with theodolites that possess automatic angle readout called as the total station (electronic tacheometers) also called as a field to finish systems.
  •  These record distance and angles simultaneously.

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