---
title: "Accuracy and precision defined"
subject: "Surveying I"
chapter: 2
key: "semester_3/surveying_i:accuracy-and-precision"
source: https://www.bctengineer.com/q/semester_3/surveying_i/accuracy-and-precision
site: BCT Engineering
---

# Accuracy and precision defined

Surveying I · Chapter 2: Distance Measurements

- Asked in: 2081 Bhadra, 2 marks; 2079 Bhadra, 1 marks (asked 2 times)
- Video: [Accuracy vs Precision | Why Surveyors Need Both](https://www.youtube.com/watch?v=D6BDO1sgupk) — Offshore Surveys
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/accuracy-and-precision
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-2

## Question

Describe the terms accuracy and precision used in surveying with examples.

## Short answer

Accuracy is closeness to the true value; precision is repeatability. Tightly grouped readings away from the true value are precise but not accurate. Systematic errors affect accuracy, random errors affect precision.

## Answer

**Accuracy** is the degree of closeness of a measured value to the true value. It shows how correct the result is.

**Precision** is the degree of closeness of repeated measurements of the same quantity to one another. It shows how consistent the measurements are, whether or not they are close to the true value.

#### Example
The true length of a line is 100.000 m.

| Observer | Readings (m) | Mean | Comment |
|---|---|---|---|
| A | 100.02, 99.98, 100.01, 99.99 | 100.00 | Close to the true value and to each other: accurate and precise |
| B | 100.52, 100.51, 100.52, 100.53 | 100.52 | Readings agree with one another but are far from the true value (e.g. tape too short): precise but not accurate |
| C | 99.5, 100.6, 100.1, 99.8 | 100.0 | Mean is near the true value but the readings scatter: accurate on average, not precise |

A target-shooting example: shots grouped tightly but away from the bull's eye are precise but inaccurate; shots grouped tightly around the bull's eye are both accurate and precise.

High precision does not guarantee accuracy, because a systematic error (like a wrong tape length) affects all readings equally. Accuracy needs both low systematic errors and high precision.

## Check yourself

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

1. Accuracy is the closeness of
   A. Repeated readings to one another
   B. A measured value to the true value
   C. Mean to the largest reading
   D. Readings to zero
   Answer: B. Accuracy shows how correct the result is; precision shows consistency. (Key point: Definition of accuracy)

2. Readings 100.52, 100.51, 100.52, 100.53 for a true 100.000 m line are
   A. Accurate and precise
   B. Precise but not accurate
   C. Accurate but not precise
   D. Neither
   Answer: B. They agree closely with each other but are far from the true value. (Key point: Precise but not accurate)

3. Why can high precision exist without accuracy?
   A. A systematic error such as wrong tape length affects all readings equally
   B. Random errors are always large
   C. Precision means true value
   D. The mean is always wrong
   Answer: A. A constant bias shifts all readings together, keeping them consistent but wrong. (Key point: Precision does not ensure accuracy)

## Related in Surveying I

- [Principles of surveying; scales used](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aprinciples-and-scales-used) — Surveying I, chapter 1
- [Two peg: closing error and precision of level](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-peg-cd-50m-e-mid-f-5m) — Surveying I, chapter 5
- [Working from whole to part; accuracy](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Awhole-to-part-and-accuracy) — Surveying I, chapter 1

## Related in other subjects

- [Accuracy, precision, sensitivity, resolution](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Aaccuracy-precision-sensitivity-resolution) — Instrumentation and Measurement, chapter 3
- [Terms used in a pitched roof](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-terminology) — Building Technology, chapter 4, asked 4×
- [Angular vs linear precision in traversing](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-angular-linear-precision-relation) — Surveying II, chapter 1, asked 2×
- [Characterizing control system by speed, accuracy, stability](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Aspeed-accuracy-stability) — Control Systems, chapter 1