---
title: "Fundamental lines of theodolite and relations"
subject: "Surveying I"
chapter: 7
key: "semester_3/surveying_i:fundamental-lines-of-theodolite"
source: https://www.bctengineer.com/q/semester_3/surveying_i/fundamental-lines-of-theodolite
site: BCT Engineering
---

# Fundamental lines of theodolite and relations

Surveying I · Chapter 7: Transit and Theodolite

- Asked in: 2059 Chaitra, 6 marks; 2062 Poush, 7 marks; 2058 Chaitra, 6 marks; 2078 Kartik, 2+2 marks (asked 4 times)
- Video: [Part 22 ! Theodolite ! Fundamental lines of theodolite ! Relationship between fundamental lines !...](https://www.youtube.com/watch?v=ZcQNTHQ8DQ0) — NARAYANA COACHING CLASSES
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/fundamental-lines-of-theodolite
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-7

## Question

What are the fundamental lines of a theodolite? Explain their relationship with neat sketches when the theodolite is in proper adjustment.

## Short answer

The fundamental lines are the vertical axis, horizontal (trunnion) axis, line of collimation, plate level axis, and altitude bubble axis. In proper adjustment, the plate level axis is perpendicular to the vertical axis, the line of collimation is perpendicular to the horizontal axis, the horizontal axis is perpendicular to the vertical axis, and the altitude bubble axis is parallel to the line of collimation.

## Answer

The **fundamental lines** of a transit theodolite are the imaginary lines and axes about which the instrument is built. They are:

1. **Vertical axis** - the axis about which the whole upper part of the instrument rotates horizontally.
2. **Horizontal (trunnion) axis** - the axis about which the telescope rotates in the vertical plane.
3. **Line of collimation (line of sight)** - the line joining the intersection of the cross hairs to the optical centre of the objective and its extension.
4. **Axis of the plate level (bubble line)** - the tangent to the plate bubble at its centre; it is horizontal when the bubble is central.
5. **Axis of the telescope level (altitude bubble)** - the tangent to the altitude bubble at its centre, used for levelling the line of sight.

#### Relationship in perfect adjustment
- The **axis of the plate level** is perpendicular to the **vertical axis**. (So the vertical axis is vertical when the plate bubble is central.)
- The **line of collimation** is perpendicular to the **horizontal axis**. (So the telescope sweeps a plane, not a cone.)
- The **horizontal axis** is perpendicular to the **vertical axis**. (So the plane swept is vertical.)
- The **axis of the telescope level** is parallel to the line of collimation.
- The **vertical axis** passes through the centre of the graduated horizontal circle, and the **horizontal axis** through the centre of the vertical circle.

```text
            line of collimation
         ---------------------->
   telescope  \
            ---O--- horizontal (trunnion) axis
               |
               |   vertical axis
               |
           plate level (axis perpendicular
           to vertical axis)
```

## Check yourself

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

1. What should be the relation of the plate level axis to the vertical axis?
   A. Parallel
   B. At 45 degrees
   C. Perpendicular
   D. Intersecting at the trunnion axis only
   Answer: C. Then the vertical axis is vertical when the bubble is central. (Key point: Plate level)

2. What should be the relation between the line of collimation and the horizontal axis?
   A. Parallel
   B. Coincident
   C. Equal in length
   D. Perpendicular
   Answer: D. Then the telescope sweeps a plane, not a cone. (Key point: Collimation)

3. What should be the relation between the horizontal axis and the vertical axis?
   A. Perpendicular
   B. Parallel
   C. Same line
   D. Coincident with the collimation line
   Answer: A. The plane swept by the telescope is then vertical. (Key point: Horizontal axis)

4. What is the relation of the axis of the telescope level to the line of collimation?
   A. Perpendicular
   B. Parallel
   C. Intersecting at 90 degrees
   D. Unrelated
   Answer: B. This allows the line of sight to be levelled. (Key point: Telescope level)

## Related in Surveying I

- [Fundamental lines violated; minimising error](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afundamental-lines-error-minimised) — Surveying I, chapter 7
- [Temporary adjustments; fundamental lines](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atemporary-adjustments-and-fundamental-lines) — Surveying I, chapter 7
- [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×
- [Closing error and graphical adjustment](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aclosing-error-graphical-adjustment) — Surveying I, chapter 4, asked 13×

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