3.2.4. Distance measurement

The EDM instruments generate the EM waves which are modulated and propagated. The EDM is kept at one point and the reflecting prism at the other point whose distance is to measured. The EDM is set up properly with temporary adjustment as has been explained for other equipment in Module 1. Many EDMs have optical plummet for accurate centering and auto levelling facility which levels the base within a certain small range. The optical plummet and auto-levelling facility save lot of time in the field for temporary adjustment of EDM. The diaphragm is properly focussed. Now, the prism is bisected accurately at its centre before taking the distance measurements from EDM.

To begin measuring distance with an EDM, it is important to first set it up correctly. The EDM needs to be positioned at one point, while a reflecting prism is placed at another location, where the distance measurement is desired. A temporary adjustment of the EDM is made to ensure that it is stable and level. Many modern EDMs come equipped with features such as an optical plummet, which helps to center the device accurately over a point on the ground, and an auto-leveling function for quick setup. The instrument's diaphragm, which helps in focusing, needs to be adjusted, and the reflecting prism must be aligned precisely with the EDM to ensure accurate measurements.

Think of setting up an EDM like preparing a camera for a perfect photograph. Just as you need to focus the camera lens and ensure it is steady before capturing a picture, the EDM must be leveled and adjusted correctly to ensure precise measurements of distance.

The EDM sends out a laser or infrared beam which is reflected back from prism to the instrument, and the instrument calculates the distance travelled by the beam. Some EDM instruments use pulsed laser emissions, and these instruments determine the distance by measuring the time taken between the transmission of the signal and the reception of the reflected signal, by taking advantage of the pulsed laser beam.

Once the EDM is set up and properly focused, it emits a laser or infrared beam towards the reflecting prism. When this beam strikes the prism, it reflects back toward the EDM. The instrument calculates the distance based on the time it takes for the beam to travel to the prism and back. Some advanced EDMs utilize a technique called pulsed laser emissions, where they send out bursts of laser light and measure how long it takes for each pulse to return. This time delay helps to determine the precise distance between the EDM and the prism.

Imagine sending a flashlight beam into the darkness and waiting to hear the echo of your voice as it bounces back from a wall. Just like how you can figure out how far away that wall is by timing how long it takes for the echo to return, the EDM does the same with the laser beam to calculate distances accurately.

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