---
title: "Theodolite construction principle; micrometer"
subject: "Surveying I"
chapter: 7
key: "semester_3/surveying_i:theodolite-construction-principle-micrometer"
source: https://www.bctengineer.com/q/semester_3/surveying_i/theodolite-construction-principle-micrometer
site: BCT Engineering
---

# Theodolite construction principle; micrometer

Surveying I · Chapter 7: Transit and Theodolite

- Asked in: 2075 Chaitra, 4 marks; 2072 Chaitra, 4 marks (asked 2 times)
- Video: [Theodolite |  Parts of Theodolite  | Types of Theodolite  | Unit 2 Surveying & Positioning](https://www.youtube.com/watch?v=7AJ6UsCHtMw) — Education Point Online
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/theodolite-construction-principle-micrometer
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-7

## Question

Explain the construction principle of the theodolite and the function of the micrometer screw in an optical theodolite (and the uses of theodolite).

## Short answer

A theodolite consists of a levelling head, lower plate with horizontal circle, upper plate carrying verniers and standards, and a telescope on a horizontal axis with a vertical circle. Horizontal angles are differences of readings when sighting points. The micrometer screw rotates a plane-parallel plate to shift the circle image for precise minute and second reading.

## Answer

A **theodolite** is an instrument for measuring horizontal and vertical angles. A telescope that can turn about a vertical axis and a horizontal axis is sighted on a point, and the angular movement is read on graduated circles.

#### Construction
```text
        eyepiece   telescope   objective
         \______________/
            |    ||  vertical circle
         ---+----++---  horizontal axis
         |  standards (A-frame)  |
         ======= upper plate (verniers) ======
         ======= lower plate (graduated circle) ===
              levelling head, 3 foot screws
                   tripod
```
- **Levelling head** with three foot screws, resting on the tripod.
- **Lower plate** carrying the graduated horizontal circle (0 to 360 degrees), with a lower clamp and tangent screw.
- **Upper plate** carrying the verniers (or micrometer), plate levels, and the standards; upper clamp and tangent screw.
- **Telescope** (internal focusing) with cross hairs, fitted to the horizontal axis, with altitude level.
- **Vertical circle** fixed to the telescope, with its vernier on the T-frame.

#### Principle
The telescope turns in a horizontal plane about the vertical axis and in a vertical plane about the horizontal axis. The difference of the circle readings when the telescope is sighted on two points gives the angle between them.

#### Function of the micrometer screw (optical theodolite)
The micrometer screw turns an optical plane-parallel plate which shifts the image of the circle graduation. It is turned until the index mark falls exactly on the nearest main graduation. The graduated drum on the screw gives the fraction of the smallest division (minutes and seconds). It replaces the vernier, with a least count of up to 1 second or less.

#### Uses of a theodolite
Measuring horizontal and vertical angles; setting out angles; ranging a line; prolonging a line; measuring distances by stadia tacheometry; finding heights; running traverse and triangulation; setting out buildings, roads and bridges.

## Check yourself

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

1. What is the principle of the theodolite?
   A. Distance is read off a staff
   B. Magnetic bearings are read
   C. The telescope turns about vertical and horizontal axes, and the difference of circle readings gives the angle
   D. Heights are got by a level bubble
   Answer: C. Angle is the difference between two readings. (Key point: Principle)

2. What does the lower plate carry?
   A. The verniers
   B. The telescope
   C. The plate levels
   D. The graduated horizontal circle
   Answer: D. The upper plate carries the verniers. (Key point: Lower plate)

3. What is the function of the micrometer screw?
   A. Moves a plane-parallel plate to read the fraction of the smallest division
   B. Levels the board
   C. Clamps the lower plate
   D. Focuses the objective
   Answer: A. It replaces the vernier with a least count of about a second. (Key point: Micrometer)

4. Which is a use of a theodolite?
   A. Measuring only the heights of stations
   B. Setting out angles and prolonging lines
   C. Plotting directly on a sheet
   D. Only measuring dip
   Answer: B. It measures and sets out horizontal and vertical angles, etc. (Key point: Uses)

## Related in Surveying I

- [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×
- [Optical-reading repeating and directional theodolites](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aoptical-repeating-and-directional-theodolite) — Surveying I, chapter 7

## Related in other subjects

- [Types of optical fibre: construction and propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-fiber-types-construction) — Propagation and Antenna, chapter 6, asked 2×
- [Construction and types of optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-construction-types) — Propagation and Antenna, chapter 6
- [Principle of optical distance measurement](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aoptical-distance-measurement-principle) — Surveying II, chapter 2
- [Optical distance principle, tangential horizontal distance](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aoptical-principle-tangential-horizontal-distance) — Surveying II, chapter 2