ELECTRONIC DISTANCE MEASUREMENT ( EDM)

Electronic distance measurement is surveying equipment that is used for measuring the distance between two points. It is also known as Electromagnetic distance measurement. The measurement is done by using electronic methods. For more articles CLICK HERE

METHODS OF MEASURING DISTANCE

Different methods are adopted for measuring the distance between any two given points. Three methods used are

• Direct distance measurement
• Optical distance measurement
• Electromagnetic distance measurement 

DIRECT DISTANCE MEASUREMENT

The direct distance measurement is actually measured on the ground. Chain or tapes are used for this method. Other instruments which are used are

• Passometer
• Pacing
• Chaining
• Odometer
• Speedometer

Chaining can be used for work of normal precision and for higher precision tapes can be used. Chaining is the basis of surveying.

OPTICAL DISTANCE MEASUREMENT

This method of measurement uses a tachometer or telemetric method. These are very rapid and convenient. But its precision is less compared to chaining. Although these are more suitable in steep, swap, etc.

ELECTROMAGNETIC DISTANCE MEASUREMENT (EDM)

Electronic distance measurement is surveying equipment that is used for measuring the distance between two points. The principle behind the EDM is electromagnetic waves. These rely on the generation, propagation, reflection, and reception of electromagnetic waves.

During the war in 1939-1945, the use of Radar leads to the invention of EDM. Since the radio waves were used. Thus for surveying these Electromagnetic waves are used.

EDM can be used for the measurement of horizontal and vertical angles and distances. They can automatically measure and record the information.

PRINCIPLES OF EDM.

The EDM uses the electromagnetic waves, the type of waves generated depends on various factors such as frequency, wavelength, and period. These are represented in the form of periodic sinusoidal waves.

Frequency is defined as the number of times the waves complete one cycle. It is represented in hertz(Hz).

The length traveled by the waves in one cycle is known as wavelength. It is represented in meters.

The time period is the inverse of frequency. That is the time takes to complete one cycle. It is represented by seconds.

The velocity of the electromagnetic waves depends on the medium. While the properties may vary according to the source.

f = c / λ = 1 / T

Where

c is the speed of light in a vacuum.

T is the time period 

f is the frequency

λ is the wavelength

WORKING OF EDM

The distance between two points is calculated based on the propagation of electromagnetic waves. 

Consider two points A and B, and we have to calculate the distance between them. First, a transmitter is placed at point A. Then a receiver and a timer are placed at another point B. Now an Electromagnetic wave is propagated from A to B, the timer is kept on. The time at which the receiver accepted the wave is noted and this time is called  Transit time. 

Here we know the velocity and transit time of the wave propagated. But measuring the transit time is a little tougher. Therefore a reflector is placed at B. The point a acts as a transmitter and receiver. Thus the double transit time is calculated. 

Now the distance is measured by the phase difference between the transmitted and received signals.

The distance covered by the wave is 

2D = nλ + ∆λ

Where,

D is the distance 

nλ is resolving the ambiguity of the phase comparison

∆λ is the fraction of wavelength travelled by the wave

This is solved automatically by the EDM device and the result is displayed.

CLASSIFICATION OF  ELECTRONIC DISTANCE MEASUREMENT

On the basis of the Electromagnetic waves used in the EDM. They are classified into three types. They are

• Microwave instruments
• Visible wave instruments
• Infrared instruments

These Electromagnetic waves have higher frequencies and fewer wavelengths.

MICROWAVE INSTRUMENTS

In this type, a microwave is transmitted from dipoles that are placed in the instrument. The frequency of the carrier is about 3 to 30GHz. They use frequency modulation. For the measurement of distance phase comparison is used. They have an accuracy of ±15mm to 5mm/ km.  An example of microwave instruments is Tellurometer. 

Here an electronic signal is needed to place in the reflecting end of the line.  A microwave EDM requires two instruments and two operators such as master instrument, remote instrument, reflector, passive reflection. 

VISIBLE LIGHT INSTRUMENTS

Geodimeter is an instrument that uses visible light. The frequency of these type o instruments is 5*10^14Hz. Corner cube prisms are used as reflectors. The carriers transmit the visible lights by using mirrors or lens. Hence the frequency falls off with the distance the range o these instruments is less compared to the microwave EDM.

Amplitude modulation is used for measuring distance.  It uses only one instrument. Using the same carrier the line is measured by three different wavelengths. They produce a high accuracy of ±0.2mm to ±1mm. It also has a range of 2 to 3 km.

INFRARED INSTRUMENTS

Infrared instruments use gallium arsenide diode for their work. Since they have the ability to emit infrared. They are inexpensive and can be easily modulated by amplitude modulation. The wavelength is about 0.9micrometers.

 These can be mounted in a theodolite because they are light and compact. Since these infrared waves come near the bandwidth of visible light. The corner cubes are also used at the reflector sides. The power output of the diode is low. So the range of the instrument is about 2 to 5 km. Wild diplomats use this type of wave for measuring. The accuracy for them ar about ±10mm.

ADVANTAGES OF ELECTRONIC DISTANCE MEASUREMENT

When compared to other measuring methods EDM has many advantages. Since it is an electronic device manual work power is less. Horizontal and vertical angles, Horizontal and vertical distance are automatically measured and recorded in EDM.

• High accuracy.
• Field work can be done fastly
• Reduces errors.
• Less manual work 
• Obstacles to chaining can be over comes
• Less calculation needed and more precise
• Convenient and reliable methods.

For details about

DAMS AND ITS TYPES click here

RETAINING WALL AND ITS TYPES click here 

USES OF GEOTEXTILES click here