Understanding Electromagnetic Distance Measurement (EDM) Instruments

Electromagnetic Distance Measurement (EDM) is a surveying method that uses electromagnetic waves to measure the distance between two points. This technology has revolutionized surveying by providing faster and more accurate distance measurements compared to traditional chaining or taping methods.

EDM instruments work by sending an electromagnetic wave (like light or radio waves) from one point to another and measuring the time it takes for the wave to travel to a reflector at the second point and return. The distance is then calculated based on the speed of the wave and the measured travel time.

Let's look at the options provided and how they relate to EDM:

• Geodimeter: This was one of the earliest EDM instruments, developed in the 1940s. It typically used visible light or near-infrared light to measure distances. Geodimeters are indeed used for measuring distances using electromagnetic waves.
• Tellurometer: Developed around the same time as the Geodimeter, the Tellurometer used microwaves for distance measurement. It was particularly useful for longer distances and in conditions where visibility was poor, like foggy weather. Tellurometers are classic examples of EDM instruments.
• Distomats: This term refers to modern electronic distance measuring instruments, often integrated into total stations. 'Distomat' was a popular trade name used by Wild Heerbrugg (now Leica Geosystems) for their EDM instruments. Modern EDM instruments, including those referred to as Distomats, use infrared light or laser beams. They are widely used today for precise distance measurement.

Based on the descriptions above, all the instruments listed — Geodimeter, Tellurometer, and Distomats — are instruments or systems that utilize Electromagnetic Distance Measurement principles to determine the distance between points.

Therefore, if Geodimeters, Tellurometers, and Distomats are all types of EDM instruments used for measuring distance, the correct answer includes all of them.

Revision Table: EDM Instruments

Additional Information on Distance Measurement in Surveying

Distance measurement is a fundamental aspect of surveying. While EDM is widely used today, other methods include:

• Chaining/Taping: Direct measurement using a chain or tape. This is a traditional method prone to errors over long distances or on uneven ground.
• Tacheometry: Indirect measurement of distance using optical principles, often with a theodolite and stadia hairs.
• GPS/GNSS: Using satellite signals to determine the coordinates of points, from which distances can be calculated. This is also a form of electromagnetic measurement, but typically used for positioning rather than direct line-of-sight distance measurement between two points on the ground surface in the same way as typical EDM instruments.

EDM instruments offer significant advantages like high accuracy, speed, and the ability to measure over challenging terrain, making them indispensable tools in modern surveying.