3.2.1. Principle of EDM

The measurement of distances requires the EDM unit (called transmitter) which transmits Electro-magnetic (EM) wave, and a reflecting prism (called receiver/reflecting unit). The slope distance is determined between the points on the ground where these two units can be kept apart (Garg, 2021). The EDM is kept at a point known as master station and reflecting prism at another point called remote station.

The Electronic Distance Measurement (EDM) system consists of two primary components: a transmitter (the EDM unit itself) and a receiver (the reflecting prism). The EDM unit emits electromagnetic waves which travel to the prism and are reflected back. The key role of the prism is to reflect the waves back to the EDM unit accurately. The distance measured is the slope distance between these two units.

Imagine sending a flashlight beam across a room to a mirror and then back to you. The distance from you to the mirror can be estimated by measuring how long it takes for the light to travel there and back. Similarly, the EDM uses light (in the form of electromagnetic waves) to measure large distances.

The general principle used in EDM is that a modulated EM beam is transmitted from a transmitter kept at the master station to a reflector which is kept at the remote station, and receiving the beam back at the master station. The instrument measures the travel time of EM wave from transmitter to receiver and back to transmitter. The slope distance between transmitter and receiver is computed by taking half of travel time multiplied by the velocity of EM wave (i.e., the velocity of light).

The EDM calculates distance by measuring how long it takes for the electromagnetic wave to travel to the prism and back. By noting the travel time, the EDM multiplies it by the speed of light, then divides by two (since the time includes both the trip to the prism and the return trip). This method allows for precise distance measurements, even over large distances.

Think of it as timing how long it takes you to throw a ball to a friend and back. If the ball takes 2 seconds to come back, the total journey took 2 seconds, but the distance you need to measure is only to your friend, so you'd divide the total time by 2.

Since measurement of time requires very precise observation, so in modern EDMs, the distance is determined by modulating the continuous carrier waves at different frequencies, and then measuring the phase difference at the master station between the outgoing and the incoming signals.

Modern EDMs use a technique called phase difference measurement for increased accuracy. Instead of merely timing the signal's travel time, the EDM sends out a carrier wave that is modulated at various frequencies. By measuring the phase shift of this wave when it returns to the transmitter, it can determine more precisely the distance to the reflector.

Imagine you’re at a concert. If you hear the sound of the drum beats at different intervals due to the music echoing in the hall, you could use this timing difference to pinpoint the location of the drummer more accurately by knowing the timing of the beat and the speed of sound.

The phase shift method is considered to be the most accurate one as it allows a very narrow beam, but its measuring range is limited. Phase shift is typically measured in degrees where a complete cycle is 360º, and the wave form repeats every 360°. As the EM wave travels in a sinusoidal wave form, the distance between two wave form peaks is known as the wavelength (λ).

In the phase shift method, the EDM determines the distance by observing how much the wave has shifted in phase as it travels to the prism and back. The wave moves in a cyclical pattern, completing one cycle in 360 degrees. The distance to the target is computed by averaging multiple readings and comparing the phase shift experienced during the round trip, using the known wavelength of the EM wave.

Think about a wave in the ocean. If you observe the way the waves come in and go back out, you can measure the distance between the peaks of the waves to understand how far the waves have traveled. Similarly, the EDM measures the 'wave peaks' to find distance.

The distance (D) is then determined as one-half of the sum of the number of wavelengths (n) in the double path distance multiplied by the wavelength (λ) plus the partial wavelength (p) represented by the phase difference. The distance is determined by the equation: D =½ (n λ + p) (3.2)

The final distance calculation accounts for both complete wavelengths and any partial wavelengths captured during measurements. It combines these factors to give an accurate representation. The formula D = ½ (nλ + p) simply states that you take the total signal travel distance (measured as the number of complete wavelengths plus the partial wavelengths) and divide by two to get the actual distance from the EDM to the reflector.

Imagine a bicycle ride where you count complete laps around a track (complete wavelengths) and then add any extra distance you traveled in a straight line before stopping (partial wavelength). You combine those distances to find out how far you rode in total.

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