What is the Electronic Distance Measurement (EDM) in surveying–different types of instruments in (EDM) , Geodimeters, Tellurimeter & Total station instruments in surveying.

 Electronic Distance Measurement (EDM)  —

Electronic distance measuring instruments measure distance by electromagnetic energy between two points.

Basic concept —    

Time taken for an alternating current to go through one complete cycle of values is called period of the wave. One cycle of the wave motion is completed and the number of cycles per unit of time is called frequency. The unit of frequency is hertz (Hz) which is one cycle per second The linear length  γ  of a wave is wavelength which is a function of frequency f and the velocity of electromagnetic radiation C as γ  =  C/f.

 Oscillators convert DC drawn from an energy source into an AC of continuous sine waves.The process of superimposing the desired sine wave or other periodic signal on to the carrier wave is called modulation.

Three main types of modulation  —

(1) Amplitude modulation 

(2) Frequency modulation 

(3) Phase modulation.

In amplitude modulation, the frequency and the phase of the carrier wave do not change but the strength and amplitude Vc of the carrier wave, V =Vc  sin Wc. t alternates sinusoidally degree of modulation. It is defined as V'/Vc where V'c is the amplitude of modulation wave.

In frequency modulation the amplitude of the carrier wave is kept constant but the frequency varies according to the amplitude and polarity of the modulation signal. The carrier frequency is increased during one half cycle of the modulation signal and decreased during the other half cycle; thus the frequency is least positive and highest when the modulation is most positive.

In phase modulation as in frequency modulation the amplitude of the carrier wave remains constant but the phase of the carrier wave is varied according to the phase of the modulation wave. 

Classification of Electromagnetic Radiation —

Total spectrum of electromagnetic radiation used in electronic and electro optical distance measurements encompasses wavelengths from the visible light of about 5(10-7) to about 3(104)m at the radio frequency region. Frequencies are large numbers and always negative powers of the basic unit.

(a) Modulation Wave 

(b) Carrier wave 

(c) Frequency modulate carrier wave (frequency changes)

 (d) Phase modulate carrier wave (phase changes)

Basic principle of EDM  —

In measuring the distance between two points electronically, an alternating signal travels from one point to the other, it is reflected or returned in same manner then is compared with the phase of the original signal to determine the travel time for the round trip. If the distance is to be measured by direct time comparison with an accuracy corresponding to the nearest cm, a time of approximately 67 (10-12) Sec must be distinguishable. This interval of time is difficult to measure directly but it can be resolved by a phase measurement of a signal with a period for direct transmission, so a much higher carrier frequencies employed and the signal appears as a modulation frequency. Depending on the type of carrier wave employed, EDM instrument can be classified as, ——

Basic concept of EDM 

 1) Microwave instrument 

2) Visible light instrument

3) Infrared instrument

Light frequencies permit the use of optical corner reflectors at the section stations but require an optically clear path between two stations. Microwave systems can operate through fog and clouds although an optically clear path is required if the vertical angle between two stations must be determined to convert into a horizontal distance. The presence of fog or clouds may cause a loss in accuracy and prevent a reliable estimate.

Name four EDM devices. Answer: —

1. Micro wave instruments 

2. Infrared wave instruments

 3. Light wave instruments 

4. Radio wave instruments

 Different Types of Instruments (EDM) —

(1) Geodimeters —

All geodimeters employ visible 'light as the carrier. The measuring set consists of an active transmitter and receiver at one end of the line to be measured and a passive retro directive prism reflector at the other end. Continuous light emission in the transmitter is intensity (amplitude) modulated, using a precision radio frequency generator and an electro-optical shutter to form sinusoidal light intensity waves. The distance is obtained by comparing the phases of outgoing modulation waves with those received by the receiving component after reflection from the distant reflector. All reflectors in the modern instruments are based on the retro directivity principle. Each unit in the reflector is a retro directive prism made of three mutually perpendicular reflecting surfaces.

(2) Tellurimeters  —

The tellurimeter uses microwaves at about 3, 10 or 35 GHz as the carrier. The measuring set consists of two active units with a transmitter or receiver; one is called the master and the other is called the remote unit. The carrier frequencies of the two units differ slightly making it possible to utilize intermediate frequency amplification. Because the carriers are microwaves and the beam widths are narrow between 2 and 20°. Measuring can be carried on either at night or day time, through haze or light rain, although heavy rainfall may reduce the working range. The bare out lines of the measuring principle consists of a frequency modulated carrier wave from the master station being sent to the remote station where it is received or transmitted to the master station. There the phase difference between the transmitted and received modulation or pattern waves is compared. Knowing the phase difference or by decade modulation technique distance can be determined.

Hewlett-Packard 3800: This is a modern instrument. Block diagram is shown in Figure below. The transmitter uses a GaAs diode which emits amplitude modulated (AM) infrared light. Frequency of modulation is precisely controlled by a crystal oscillator.. The intensity or amplitude variation is properly represented by sine waves. Environmental correction factor can be directly dialed into the transmitter to slightly vary the frequency so that a constant wavelength is maintained despite atmospheric variation. Hence no adjustment of distance is necessary at a later stage.

(3) Total station instruments  —

Modern surveying system typically consists of an electronic total station, electronic field book and software used in the office for processing data.

The total stations function is to measure horizontal and vertical angles and slope distances in a single integrated unit. It is usually connected to an electronic field book. The field system (total station and field book) is usually controlled via the field book. The principal reason is that the field book keyboard does not transfer the force used to press the keys to the total station.

In operation the total station is set up over the required point and its height over the survey station measured. Then the operator points at a prism/target and initiates POTE reading. Usually this is done by pressing a key on the field book. In some system it may be a key on the electronic total station. 

While the basic data sent by the electronic total station consist of slope distance, horizontal and vertical angle, other data may be included in the data stream. This may include units setting, parts per million (ppm) value, prism constant being used, etc. Additionally, calculated values such as coordinates, azimuths, and horizontal distances may be transmitted. Electronic total stations can also have a variety of functions to improve efficiency and accuracy.

Some of these may be corrections for collimations, curvature and refraction and horizontal and vertical angles to compensate for the title of the vertical axis. The electronic field book's basic function is to store the raw data gathered in the field,

including horizontal and vertical angles, slope distances, heights and instruments and targets, temperature and pressure, point numbers and descriptive codes.

One of the most crucial aspects of the electronic data collection concept is data flow. Traditional surveying techniques force one to view surveying as a data gathering activity. This view does not recognize the fact that once a design is completed based on the survey data there usually is a need to transfer this design on to the topography. In modern surveying, setting out is a given equal importance as data gathering.

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