3.2.2. EDMs classification based on range

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 are categorized based on the type of electromagnetic waves they use and their respective measurement ranges.

• High range equipment, which utilizes radio waves, can measure distances up to 100 km. This is useful for large-scale projects where long distances need to be measured quickly and accurately.
• Medium range devices employ microwaves and are capable of measuring distances up to 25 km, suitable for medium-scale surveys.
• Short range EDMs use infra-red or visible light for distances up to 5 km, typically used in more localized projects. This range makes them ideal for detailed construction and engineering work in confined areas.

Think of measuring the distance from your home to a distant city - you would need a different tool than measuring the distance to your neighbor's house. For long distances, like the city, a tool that can reach far away is essential (high range). For your neighbor, a simpler tool would suffice (short range). Similarly, engineers choose specialized EDMs for the task at hand, depending on how far they need to survey.

Sunlight or electromagnetic spectrum consists of different wavelengths at different frequency. Various wavelength regions of EM waves are shown in Figure 3.4. The EDMs are now-a-days incorporated in electronic theodolites (or Total Stations) that can automatically measure vertical angle to compute horizontal and vertical distances. The present EDM instruments have several other useful features; laser plummet, 30x magnification, high resolution LCD display, upload and transfer of data, data editing and exchange, and bluetooth connectivity.

The electromagnetic spectrum encompasses a range of wavelengths from sunlight, which forms the foundation of how EDMs operate. Different EDM devices utilize various parts of this spectrum to measure distances. Modern EDMs are integrated into Total Stations for enhanced functionality, allowing automatic measurement of angles and distances. They also come with useful features such as laser plummets for precise alignment, high magnification for better visibility, and capabilities to engage with digital platforms for data management. This integration improves efficiency and accuracy in surveying operations.

Just as a smartphone combines multiple features like a camera, GPS, and a communications device into one handy gadget, modern EDMs combine distance measurement with various technological features. Imagine trying to navigate a new city with only a paper map versus having a GPS-enabled device that guides you with real-time directions and traffic updates—EDMs are the advanced navigators for surveyors, enabling them to work efficiently on site.

The microwave-based EDMs make use of microwaves, such as Tellurometers, in the wavelength from 10 cm and 3 cm. The range of these instruments is up to 100 km with an error of ±5 to 15 mm/km. They can also be used in night and in adverse weather conditions. They consist of two identical units; where one unit is used as master unit and the other as remote unit. The master unit can be converted into a remote unit and a remote unit into a master unit, if required. Each unit is kept at two ends of a line to determine the distance. For long distance measurement, a communication facility is provided with each unit to interact with survey team members during the measurements.

Microwave-based EDMs, like the Tellurometer, use specific wavelengths of microwaves for accurate long-distance measurements. These instruments can measure distances up to 100 km with a small margin for error, which is crucial for precision in surveying. One unique feature is the ability to switch roles between the master and remote units, allowing flexibility during surveys. Communication capabilities further enhance coordination among the surveying team during measurements, making these tools effective for various conditions, including nighttime or inclement weather.

Consider using walkie-talkies to communicate with a friend while you’re both racing each other across a large field. Excellent for long distances, but you need to stay in touch to make sure you’re both on the same page—this is similar to how microwave-based EDMs operate, with one unit acting as the leader (master) sending signals to the follower (remote), allowing for accurate and coordinated distance measurements across vast areas.

Visible light or infrared wave-based EDMs rely on propagation of modulated waves, such as Geodimeter or Distomat (Gopi et.al., 2017). The instrument is kept at one end and the reflecting prism at the other end of a line. These instruments are light and economical, and can also be mounted on the top of a theodolite. Such EDMs are very useful for civil engineering projects, as the accuracy of these measurements varies from 0.2 mm/km to 1 mm/km distance. They can be used to measure distance up to 5 km.

Infrared and visible light EDMs utilize light waves to facilitate distance measurement. Instruments like the Geodimeter or Distomat reflect light off a prism to calculate distances, making them lightweight and cost-effective options for surveyors. They possess high accuracy, typically ranging from 0.2 mm/km to 1 mm/km, and are effective for distances up to 5 km, making them particularly suitable for civil engineering applications where precision is crucial.

Think of how a flashlight works; it sends out a beam of light that can reflect off a surface leading you to know how far away something is in a dark room. Similarly, infrared EDMs shine a light (literally!) that bounces back from a prism, allowing surveyors to measure distances with impressive accuracy without needing heavy equipment.

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