---
title: "Principles of EDM measuring distances"
subject: "Surveying I"
chapter: 2
key: "semester_3/surveying_i:edm-principles"
source: https://www.bctengineer.com/q/semester_3/surveying_i/edm-principles
site: BCT Engineering
---

# Principles of EDM measuring distances

Surveying I · Chapter 2: Distance Measurements

- Asked in: 2081 Bhadra, 4 marks; 2080 Baisakh, 4 marks; 2079 Bhadra, 1+3 marks; 2078 Kartik, 4 marks; 2076 Chaitra, 4 marks; 2076 Asoj, 4 marks; 2075 Asoj, 4 marks; 2075 Chaitra, 4 marks; 2072 Chaitra, 4 marks; 2069 Chaitra, 4 marks; 2068 Baisakh, 4 marks; 2067 Asar, 4 marks (asked 12 times)
- Video: [Principle of EDM | working Principle of Electronic Distance Meter | Understanding EDM in Hindi](https://www.youtube.com/watch?v=KWe0SAhBSI4) — EASY CIVIL
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/edm-principles
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-2

## Question

What is EDM? Explain the principles of electronic distance measurement (EDM) for measuring distances.

## Short answer

EDM measures distance using the travel time or phase shift of a modulated electromagnetic wave between instrument and reflector. Distance is D=vt/2 (pulse) or D=(N+φ/2π)λ/2 (phase, multiple frequencies resolve N). Atmospheric corrections are applied.

## Answer

**EDM (Electronic Distance Measurement)** is the measurement of a distance using the velocity and the travel time (or phase change) of a modulated electromagnetic wave (light, infrared or microwave) sent from an instrument to a reflector and back.

#### Principle
The instrument at A transmits a modulated wave to a reflector (prism or transponder) at B. The reflected wave returns to A. The distance is found from the time taken or from the phase shift of the returned wave. The wave travels with a known velocity $v = c/n$, where $c = 299{,}792{,}458$ m/s is the velocity in vacuum and $n$ is the refractive index of the air.

```text
  EDM at A ------ outgoing wave ------> Prism at B
        <------ reflected wave ------
           Double path = 2D
```

#### 1. Pulse (time-of-flight) method
A short pulse of energy is sent and the time $t$ between sending and receiving it is measured.

$$
D = \frac{v\,t}{2}
$$

It needs a very accurate clock: a time error of 1 ns gives about 15 cm error in distance. It is used for long ranges and in reflectorless laser total stations.

#### 2. Phase-comparison method
A continuous carrier wave is modulated with a fixed frequency $f$ (wavelength $\lambda = v/f$). The phase difference $\phi$ between the transmitted and the received wave is measured. The double path $2D$ contains $N$ complete wavelengths plus a fraction:

$$
2D = N\lambda + \frac{\phi}{2\pi}\lambda
\quad\Rightarrow\quad
D = \frac{1}{2}\left(N + \frac{\phi}{2\pi}\right)\lambda
$$

The instrument measures the fraction, but the whole number $N$ is unknown. It is found by using several modulation frequencies (a coarse and a fine wavelength), so $N$ is resolved automatically. This is the common method in total stations.

#### Steps in operation
1. Set up the EDM/total station over one point and a reflector over the other point.
2. Enter the temperature, pressure and prism constant.
3. Sight the reflector and press measure.
4. The instrument corrects for the atmosphere and gives the slope distance, horizontal distance and height difference.

EDMs give high accuracy (typically $\pm$(a few mm + 1 to 5 ppm)), speed and long range.

## Check yourself

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

1. What does EDM measure the distance from?
   A. Angles subtended by a bar
   B. Travel time or phase change of a modulated electromagnetic wave sent to a reflector and back
   C. Readings on a levelling staff
   D. Number of tape lengths
   Answer: B. EDM uses the velocity and time (or phase) of a modulated wave travelling to the reflector and back. (Key point: Definition of EDM)

2. In the pulse method D = v t / 2. Why is t divided by 2?
   A. The wave slows down on return
   B. The measured time covers the double path (out and back)
   C. Half the energy is lost
   D. The prism halves the time
   Answer: B. t is the round-trip time, so the one-way distance is half of v t. (Key point: Pulse method)

3. Why does the pulse method need a very accurate clock?
   A. Light travels slowly
   B. A 1 ns timing error gives about 15 cm distance error
   C. The prism delays the signal by days
   D. Temperature stops the clock
   Answer: B. Light covers about 30 cm per ns, so 1 ns error is about 15 cm one-way. (Key point: Clock accuracy)

4. In the phase-comparison method, how is the unknown whole number N of wavelengths resolved?
   A. Measured directly by the phase detector
   B. By using several modulation frequencies (coarse and fine)
   C. By using a longer tape
   D. By ignoring it
   Answer: B. The phase gives only the fraction; coarse and fine wavelengths together remove the integer ambiguity. (Key point: Ambiguity of N)

## Related in Surveying I

- [EDM operation principles and errors](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aedm-operation-and-errors) — Surveying I, chapter 2, asked 2×
- [True horizontal distance from slope distances](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aslope-distances-tape-2cm-long) — Surveying I, chapter 2
- [Distance measurement on sloping ground](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Adistance-on-sloping-ground) — Surveying I, chapter 2
- [Triangulation; EDM principle for trilateration sides](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atriangulation-and-edm-principle-trilateration) — Surveying I, chapter 8
- [Correct horizontal distance 630 m, tape 8.9 N](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atape-630m-8-9n-30-005) — Surveying I, chapter 2
- [Principle applied in two-way linear measurement](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-way-linear-measurement-principle) — Surveying I, chapter 1

## Related in other subjects

- [Principle of optical distance measurement](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aoptical-distance-measurement-principle) — Surveying II, chapter 2
- [Types of sight distances](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asight-distance-types) — Transportation Engineering I, chapter 3, asked 2×
- [Types and sources of measurement errors](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Atypes-sources-of-errors) — Metrology, chapter 2
- [Measurement tool to test an antenna as DUT](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aantenna-dut-measurement-tool) — RF and Microwave Engineering, chapter 8