---
title: "EDM operation principles and errors"
subject: "Surveying I"
chapter: 2
key: "semester_3/surveying_i:edm-operation-and-errors"
source: https://www.bctengineer.com/q/semester_3/surveying_i/edm-operation-and-errors
site: BCT Engineering
---

# EDM operation principles and errors

Surveying I · Chapter 2: Distance Measurements

- Asked in: 2078 Bhadra, 4 marks; 2074 Chaitra, 4 marks (asked 2 times)
- Video: [EDM In Survey क्या है| EDM~Electronic distance measurement in hindi|EDM Working|Types of EDM](https://www.youtube.com/watch?v=XdMhu02FicI) — PK Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/edm-operation-and-errors
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-2

## Question

What are the principles of actual operation of EDM? Describe the sources of errors of EDM.

## Short answer

EDM sends a modulated electromagnetic wave to a reflector and measures the round-trip time (pulse method D = v t / 2) or phase shift (phase-comparison method D = ½(N + φ/2π)λ). Errors include constant errors (zero error, centring) and proportional errors, predominantly from incorrect atmospheric temperature, pressure and humidity correction.

## Answer

#### Principles of operation of EDM
An EDM instrument sends a modulated electromagnetic wave (infrared, laser or microwave) to a reflector at the other end of the line. The reflected wave returns to the instrument, and the distance is obtained by measuring either the **time** taken or the **phase difference** of the wave.

1. **Pulse method:** $D = \dfrac{v\,t}{2}$, with $v = c/n$ the velocity in air and $t$ the round-trip time.
2. **Phase-comparison method:** the carrier wave is modulated with frequency $f$ ($\lambda = v/f$). The phase shift $\phi$ is measured and
$$
D = \frac{1}{2}\left(N + \frac{\phi}{2\pi}\right)\lambda
$$
The integer $N$ is solved by using two or more modulation frequencies. A microprocessor applies the atmospheric corrections and displays the distance.

#### Sources of errors in EDM
The final error is usually expressed as $\pm(a\ \text{mm} + b\ \text{ppm})$.

**Distance-independent errors (constant)**
- Zero error (instrument/reflector constant), due to the difference between the electrical centre and the mechanical centre.
- Centring errors of the instrument and the reflector over the stations.
- Phase measurement (resolution) error.
- Error in measuring the height of the instrument and the target.

**Distance-dependent errors (proportional)**
- Error in the atmospheric refractive index from wrong temperature, pressure and humidity readings. This is the largest source.
- Error in the modulation frequency (instrument oscillator drift).
- Error in the velocity of light used.

**Other errors**
- Cyclic error (phase non-linearity) inside the instrument.
- Reflection from other objects (ground swing) and multipath.
- Poor line of sight, beam passing close to the ground or heat shimmer.
- Operator mistakes, e.g. wrong prism constant.

Errors are reduced by regular calibration, using accurate temperature and pressure, careful centring, and taking several readings.

## Check yourself

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

1. Which is a distance-independent (constant) EDM error?
   A. Refractive index error
   B. Zero error and centring errors
   C. Modulation frequency drift
   D. Velocity of light error
   Answer: B. Zero error, centring and phase resolution do not grow with distance (the 'a mm' part). (Key point: Distance-independent errors)

2. Which is the largest distance-dependent (ppm) source?
   A. Error in atmospheric refractive index from wrong temperature, pressure and humidity
   B. Prism height
   C. Operator booking
   D. Zero error
   Answer: A. Atmospheric refractive index errors change the wave velocity and scale with distance. (Key point: Distance-dependent errors)

3. How is the final EDM error usually expressed?
   A. +/-(a mm + b ppm)
   B. Percent of angle
   C. +/- 1/1000
   D. In seconds
   Answer: A. A constant part (mm) plus a proportional part (ppm of distance). (Key point: Error expression)

4. In D = 1/2 (N + phi/2pi) lambda, how is the integer N obtained?
   A. From two or more modulation frequencies
   B. From the prism constant
   C. From temperature
   D. From the tape
   Answer: A. Using several frequencies resolves the whole number of wavelengths automatically. (Key point: Phase method)

## Related in Surveying I

- [Principles of EDM measuring distances](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aedm-principles) — Surveying I, chapter 2, asked 12×
- [Sources of errors in chaining/taping](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aerrors-in-taping-sources-directions) — Surveying I, chapter 2, asked 4×
- [Face observation errors; optical theodolite](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aface-observation-errors-optical-theodolite) — Surveying I, chapter 7, asked 2×
- [Local attraction: errors and elimination](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Alocal-attraction-sources-elimination) — Surveying I, chapter 4

## Related in other subjects

- [Types and sources of measurement errors](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Atypes-sources-of-errors) — Metrology, chapter 2
- [Errors encountered during file operation](https://www.bctengineer.com/question.md?key=semester_3%2Fobject_oriented_programming%3Afile-errors) — Object Oriented Programming, chapter 8
- [Testing principles and testability](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Atesting-principles) — Software Engineering, chapter 8
- [Hydro and thermal power sources](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ahydro-thermal-sources) — Power System Analysis I, chapter 1