---
title: "EDM classification; electromagnetic energy"
subject: "Surveying I"
chapter: 2
key: "semester_3/surveying_i:edm-classification-em-energy"
source: https://www.bctengineer.com/q/semester_3/surveying_i/edm-classification-em-energy
site: BCT Engineering
---

# EDM classification; electromagnetic energy

Surveying I · Chapter 2: Distance Measurements

- Asked in: 2081 Baisakh, 4 marks
- Video: [EDM | Classification of EDM | Principle of EDM | Prashant YT | Surveying 2nd | BE Civil |](https://www.youtube.com/watch?v=5coS06MeHDU) — Prashant YT
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/edm-classification-em-energy
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-2

## Question

Write the classification of EDM and the significance of electromagnetic energy.

## Short answer

EDM is classified by carrier: microwave (Tellurometer), visible light (Geodimeter), infrared (total stations), and laser (reflectorless). Electromagnetic energy propagates at known speed, can be modulated, reflected by prisms, and its velocity varies with the atmosphere, so temperature, pressure and humidity corrections are needed.

## Answer

#### Classification of EDM
EDM instruments are classified according to the type of carrier wave used.

| Type | Carrier | Range | Example/use |
|---|---|---|---|
| Microwave | Radio waves (3 to 30 GHz) | 30 to 100 km | Tellurometer (large control surveys). Needs a master and a remote unit; works in any weather |
| Light-wave (visible) | Modulated visible light | 2 to 5 km (day), more at night | Geodimeter. Needs a reflector prism |
| Infrared wave | Modulated infrared | 1 to 5 km | Distomat, most total stations; used with prisms |
| Laser (reflectorless) | Laser beam | Up to about 1 km without a prism (several km with a prism) | Measures directly to walls, rock faces and other surfaces without a prism |

By measuring principle: pulse method and phase-comparison method.
By range: short (up to 5 km), medium (5 to 100 km), long (more than 100 km).

#### Significance of electromagnetic energy
- EM waves travel at a very high, known velocity ($c = 299{,}792{,}458$ m/s in vacuum, $v = c/n$ in air), so measuring the time or phase gives the distance: $D = vt/2$.
- They travel in a straight line and can be reflected by a prism, which allows round-trip measurements.
- A carrier can be **modulated** (in amplitude or frequency) so that the wavelength and phase can be measured precisely.
- The velocity changes with the atmosphere (temperature, pressure, humidity), so these are measured and used for correction.
- Different parts of the spectrum give different ranges and weather behaviour, e.g. microwaves pass through haze and rain while light needs clear visibility.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which statement fits microwave EDM such as the Tellurometer?
   A. Needs a master and remote unit, long range, works in any weather
   B. Uses prisms only up to 1 km
   C. Reflectorless laser
   D. Only at night
   Answer: A. Microwaves reach 30-100 km and are not affected by haze or rain. (Key point: Microwave EDM)

2. Most total stations use
   A. Microwave
   B. Modulated infrared with prisms
   C. Radio only
   D. X-rays
   Answer: B. Infrared instruments such as Distomat have a 1-5 km range with prisms. (Key point: Infrared EDM)

3. Why is the carrier wave modulated?
   A. So wavelength and phase can be measured precisely
   B. To slow it
   C. To avoid reflection
   D. To change colour
   Answer: A. Modulation gives a measuring wave of known wavelength. (Key point: Modulation)

4. Why are temperature, pressure and humidity entered?
   A. Velocity of EM waves changes with atmosphere, so corrections are needed
   B. To power the instrument
   C. To set the prism
   D. For angle reading
   Answer: A. Since D = v t / 2 and v depends on air, atmospheric corrections are applied. (Key point: Atmosphere effect)

## Related in Surveying I

- [Propagation of electromagnetic energy](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Apropagation-em-energy) — Surveying I, chapter 2

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- [Classification of electrical consumers and demand](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclassification-consumers-demand) — Utilization of Electrical Energy, chapter 1
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