3.2. Electronic Distance Measurement (EDM) Devices

The EDM is an electronic distance measuring device, which measures the distance from the instrument to its target through electromagnetic waves (Garg, 2021). The EDM instruments are highly reliable and convenient, and can be used to measure distances of up to 100 km. Such long distance measurement can be done by instruments, like geodimeter, tellurometer or distomat etc.

EDM devices, or Electronic Distance Measurement devices, are tools used in surveying to measure distances between a device and a target, up to 100 kilometers away. They use electromagnetic waves to do this, making them more precise and efficient compared to traditional measuring methods. The first EDMs, like the geodimeter, were created in the 1940s and utilized modulated light beams to measure distance.

Think of an EDM as a very high-tech tape measure. Just as you can use a tape measure to find out how far it is to a wall, an EDM can measure much larger distances quickly and accurately, making it easier for surveyors to get precise measurements over long stretches of land.

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.

An EDM functions on the principle that it sends out a wave from a transmitter to a reflector, typically a prism, and measures the time it takes for the wave to return after bouncing off the reflector. This time is used to calculate the distance by using the speed of light. Measurements can be affected by factors like atmospheric conditions, but modern EDMs modulate waves and measure differences in phase for greater accuracy.

Imagine sending a flashlight beam across a lake and checking how long it takes for the light to bounce back from a boat. The time taken can help you figure out how far the boat is from you, similar to how an EDM measures distances with electromagnetic waves.

The distance is calculated as: Distance (D) = (elapsed time/2) x velocity of light (3.1)

To calculate distance using EDM, the time it takes for the electromagnetic wave to travel to the prism and back is measured. Since this time includes both the outgoing and returning trip, it is divided by two. The formula uses the speed of light because EDMs rely on light waves. This calculation is what allows surveyors to determine precise distances with a high degree of accuracy.

Think of it like timing a round trip to your friend's house and back. If you measure how long the trip takes and divide by 2 to only account for the one way distance, then multiply by how fast you can drive, you can find out just how far away your friend lives.

The EDMs are also classified on the basis of their range of electro-magnetic (EM) waves (Subramanian, 2012), as: (a) High range- radio wave equipment for ranges up to 100 km (b) Medium range- microwave equipment with frequency modulation for ranges up to 25 km (c) Short range- electro-optical equipment using amplitude modulated infra-red or visible light for ranges up to 5 km.

EDMs can be categorized based on the type of electromagnetic waves they use and the distances they can measure. High-range EDMs can use radio waves to measure distances of over 100 kilometers, while medium-range uses microwaves for distances up to 25 kilometers, and short-range EDMs use visible light or infrared waves to measure distances up to 5 kilometers.

Imagine different types of flashlights: a high-range flashlight can illuminate a whole street (like a long-range EDM), while a medium-range flashlight works well in a park (medium range), and a short-range flashlight is perfect for shining under your bed (short range). Each serves a purpose based on how far they need to shine the light.

Reflecting prisms, also known as reflectors, reflect the EM wave back to the Total Station or EDM for computing the distance (Figure 3.5).

Reflecting prisms play a crucial role in EDM measurements by reflecting the electromagnetic signals back to the EDM device. The precise alignment of these prisms with the EDM is necessary for accurate measurements. They can be single prisms or grouped together to improve accuracy over longer distances.

Consider a game of catch where you throw a ball (the EM wave) to a friend, and they throw it back to you. The prism acts like your friend, returning the ball so you can measure how far it went. The accuracy of your measurement depends on how well your friend catches and throws back the ball.

The errors in EDM measurements could be due to- Personal errors, Instrumental errors, and Natural errors. A good description of errors in EDM measurements is given by Rajput (2020).

EDM measurements can be affected by a variety of errors, including personal errors (mistakes made by the operator), instrumental errors (malfunctions or inaccuracies in the EDM itself), and natural errors (environmental factors like atmospheric conditions). Understanding these errors is crucial for ensuring the accuracy and reliability of surveying measurements.

Much like a chef can ruin a recipe by mismeasuring ingredients (personal error) or using faulty utensils (instrumental error), a surveyor must carefully consider both personal and equipment-based mistakes to ensure accurate distance measurements.

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