Electronic Distance Measurement Instrument- Types, Functions & Operations

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 travels from one end of the line to the other end. As a background, there are three methods of measuring distance between two points:

DDM or Direct distance measurement – This is mainly done by chaining or taping.

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.

EDM or Electromagnetic distance measurement – The method of direct distance measurement cannot be implemented in difficult terrains. When 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 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 distance that rely on propagation, reflection and reception of electromagnetic waves like radio, visible light or infrared waves.

Sun light or artificially generated electromagnetic wave consists of waves of different lengths. The spectrum of an electromagnetic wave is as shown below:

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 wave, the distance may be calculated. However time is too short, measuring the time taken is difficult. The improved techniques use phase difference method in which the number of completed wave and incomplete wave is measured. Knowing the length of wave, distances are calculated.

Built up microprocessors provided in the instrument calculate the distances and display it by liquid crystal display (LCD).

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 equipments are incorporated along with theodolites that possess automatic angle readout called as total station (electronic tacheometers) also called as field to finish systems. These record distance and angles simultaneously.

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. Light wave 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 day as well as in night.

The range of these instruments is up to 100 km. It consists of two identical units. One unit is used as master unit and the other as 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 line to be measured. These instruments are light and economical and can be mounted on theodolite. With these instruments accuracy achieved is ± 10 mm. 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 Instruments

These instruments rely on 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 day its range is up to 3 km. Accuracy of these instruments varies from 0.5 mm to 5 mm/km distance. These instruments are also very useful for civil engineering projects.

Operations of Electronic Distance Measurement Instruments

It is essential to know the fundamental principle behind EDM to work with it. The electromagnetic waves propagate through the atmosphere based on the equation

Where ‘v’ is the velocity of electromagnetic energy in meters per second(m/sec); f is the modulated frequency in hertz (Hz) and is, the wavelength measured in meters. Mainly the waves that are propagated can be represented like a sine wave as shown in figure below.

Another property of wave called as phase of wave , is a very convenient method of small fraction of wavelength during measurement in EDM. The points A, B, C etc. represents various phase points

                                  Fig. Sinusoidal Waves

                                     

                           

Fig. Corresponding phase values

Say AB is the survey line to me measured, having a length of D. The EDM equipment is placed at ends A and B. A transmitter is placed at A and a receiver is placed at B. the transmitter lets propagation of electromagnetic waves towards B. A timer is also placed. At the instant of transmission of wave from A the timer at B starts and stops at the instant of reception of incoming wave at B. This enable us to know the transit time for the wave from the point A to B.

Fig. Transit Time Measurement Demonstration

From the transit time and known velocity, the distance can be easily measured. Now to solve the problem arise due to difficulty in starting the timer at B, a reflector can be placed as shown below instead of a receiver at B.

Fig. Transit measurement arrangement with the help of a EDM and a reflector

Measurement of distance with EDM and a Reflector

As explained let the waves get transmitted from A and reflected from B. If the received signal is out of phase by a measure of , then equivalent distance is

Thus, the distance

where n is the integral number of wavelength,  in the double path

Error in Electronic Distance Measurement Instruments

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

Instrumental Errors

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

Natural Errors

• Atmospheric variations in temperature, pressure as well as humidity. Micro wave EDM instruments are more susceptible to these.
• Multiple refraction 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.

Modern Surveying Instruments and Their Uses

Modern surveying instruments provides faster and more precise surveying than conventional instruments. Their types and uses are discussed in this article.

In conventional surveying, chain and tape are used for making linear measurements while compass and ordinary theodolites are used for making angular measurements.  Leveling work is carried out using a Dumpy level and a leveling staff. With such surveying instruments, survey work will be slow and tedious.

Hence modern surveying instruments are becoming more popular and they are gradually replacing old surveying instruments such as compass and Dumpy level. With modern surveying instruments, survey work will be precise, faster and less tedious. Some of the modern surveying instruments are discussed in this article.

Modern Surveying Instruments and Their Uses

Following are the modern surveying instruments which are used for surveying:

• Electronic Distance Measurement (EDM) Instruments
• Total Station
• Global Positioning System (GPS)
• Automatic Level

1. Electronic Distance Measurement (EDM) Instruments

Direct measurement of distances and their directions can be obtained by using electronic instruments that rely on propagation, reflection and reception of either light waves or radio waves. They may be broadly classified into three types:

a. Infrared wave instruments

b. Light wave instruments

c. Microwave instruments

a. Infrared Wave Instruments

These instruments measure distances by using amplitude modulated infrared waves.  At the end of the line, prisms mounted on target are used to reflect the waves.  These instruments are light and economical and can be mounted on theodolites for angular measurements.  The range of such an instrument will be 3 km and the accuracy achieved is ± 10 mm.

E.g. DISTOMAT DI 1000 and DISTOMAT DI 5

DISTOMAT DI 1000

It is a very small, compact EDM, particularly useful in building construction and other Civil Engineering works, where distance measurements are less than 500 m.  It is an EDM that makes the meaning tape redundant.  To measure the distance, one has to simply point the instrument to the reflector, touch a key and read the result.

b. Light Wave Instruments

These are the instruments which measures distances based on propagation of modulated light waves.  The accuracy of such an instrument varies from 0.5 to 5 mm / km distance and has a range of nearly 3 km.

Eg: Geodimeter

Geodimeter

Geodimeter is an instrument which works based on the propagation of modulated light waves, was developed by E. Bergestand of the Swedish Geological Survey in collaboration with the manufacturer M/s AGA of Swedish.  The instrument is more suitable for night time observations and requires a prism system at the end of the line for reflecting the waves.

c. Microwave Instruments

These instruments make use of high frequency radio waves.  These instruments were invented as early as 1950 in South Africa by Dr. T.L. Wadley.  The range of these instruments is up to 100 km and can be used both during day and might.

Eg. Tellurometer

Tellurometer

It is an EDM which uses high frequency radio waves (micro-waves) for measuring distances.  It is a highly portable instrument and can be worked with 12 to 24-volt battery.

For measuring distance, two Tellurometers are required, one to be stationed at each end of the line, with two highly skilled persons, to take observations. One instrument is used as a master unit and the other as a remote unit.

Just by pressing a button a master can be converted into remote unit and vice-versa.  A speech facility (communication facility) is provided to each operator to interact during measurement.

Total Station

Total Station is a lightweight, compact and fully integrated electronic instrument combining the capability of an EDM and an angular measuring instrument such as wild theodolite.

Total Station can perform the following functions:

• Distance measurement
• Angular measurement
• Data processing
• Digital display of point details
• Storing data is an electronic field book

The important features of total station are,

1. Keyboard-control – all the functions are controlled by operating key board.
2. Digital panel – the panel displays the values of distance, angle, height and the coordinates of the observed point, where the reflector (target) is kept.
3. Remote height object – the heights of some inaccessible objects such as towers can be read directly.  The microprocessor provided in the instrument applies the correction for earth’s curvature and mean refraction, automatically.
4. Traversing program – the coordinates of the reflector and the angle or bearing on the reflector can be stored and can be recalled for next set up of instrument.
5. Setting out for distance direction and height -whenever a particular direction and horizontal distance is to be entered for the purpose of locating the point on the ground using a target, then the instrument displays the angle through which the theodolite has to be turned and the distance by which the reflector should move.

Global Positioning System (GPS)

Global Positioning System (GPS) is developed by U.S. Defense department and is called Navigational System with Time and Ranging Global Positioning System (NAVSTAR GPS) or simply GPS.

For this purpose U.S. Air Force has stationed 24 satellites at an altitude of 20200 km above the earth’s surface.  The satellites have been positioned in such a way, at least four satellites will be visible from any point on earth.

The user needs a GPS receiver to locate the position of any point on ground.  The receive processes the signals received from the satellite and compute the position (latitude and longitude) and elevation of a point with reference to datum.

Automatic Level

An automatic level is a special leveling instrument used in surveying which contains an optical compensator which maintains line of sight or line of collimation even though instrument is slightly tilted.

E.g.:  Wild NAK2 Automatic level