---
title: "Temporary adjustments; fundamental lines"
subject: "Surveying I"
chapter: 7
key: "semester_3/surveying_i:temporary-adjustments-and-fundamental-lines"
source: https://www.bctengineer.com/q/semester_3/surveying_i/temporary-adjustments-and-fundamental-lines
site: BCT Engineering
---

# Temporary adjustments; fundamental lines

Surveying I · Chapter 7: Transit and Theodolite

- Asked in: 2073 Shrawan, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/temporary-adjustments-and-fundamental-lines
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-7

## Question

Explain temporary adjustments of theodolite survey. Also show the different fundamental lines of theodolite.

## Short answer

Temporary adjustments are centering over the station, levelling with foot screws and plate level, and eliminating parallax. Fundamental lines are the vertical axis, horizontal (trunnion) axis, line of collimation, plate level axis, and altitude level axis. They must satisfy perpendicularity conditions.

## Answer

#### Temporary adjustments
These are done at every set-up before observation:

1. **Setting up (centering):** fix the theodolite on the tripod and bring the plumb bob (or optical plummet) exactly over the station mark.
2. **Levelling up:** with the foot screws and the plate levels, make the vertical axis truly vertical. Set the plate level parallel to two foot screws, centre the bubble; turn 90 degrees and centre it with the third screw; repeat until the bubble stays central in all positions.
3. **Elimination of parallax:** focus the eyepiece until the cross hairs are sharp, then focus the objective on the object so that there is no apparent movement between the image and cross hairs when the eye moves.

#### Fundamental lines of a theodolite
- **Vertical axis** - about which the instrument rotates horizontally.
- **Horizontal (trunnion) axis** - about which the telescope rotates vertically.
- **Line of collimation** - the line of sight through the intersection of the cross hairs and the optical centre of the objective.
- **Axis of the plate level** and **axis of the altitude (telescope) level**.

```text
            line of collimation
         ----------------------->
   telescope \
           ---O--- horizontal axis
              |
              |  vertical axis
```

In adjustment: plate level axis $\perp$ vertical axis; line of collimation $\perp$ horizontal axis; horizontal axis $\perp$ vertical axis; altitude level axis $\parallel$ line of collimation.

## Check yourself

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

1. Which set lists the temporary adjustments of a theodolite?
   A. Collimation, trunnion axis and plate level adjustment
   B. Setting up, levelling up and elimination of parallax
   C. Reading both verniers, changing face and repetition
   D. Closing the horizon, sets and reiteration
   Answer: B. Temporary adjustments done at every set-up are centring, levelling up and parallax elimination. (Key point: Temporary adjustments list)

2. In levelling up, after centring the bubble with the plate level parallel to two foot screws, what is the next step?
   A. Turn the telescope through 180° and fix it
   B. Tighten all foot screws fully
   C. Turn the plate level through 90° and centre the bubble using the third foot screw
   D. Focus the objective
   Answer: C. The level is turned 90° and centred with the third screw; this is repeated until the bubble stays central in all positions. (Key point: Levelling procedure)

3. What is the correct order for eliminating parallax?
   A. Focus the objective first, then the eyepiece
   B. Focus the eyepiece for sharp cross hairs, then focus the objective on the object
   C. Adjust the foot screws until the image is sharp
   D. Swing the telescope to the face right
   Answer: B. Eyepiece is focused for sharp cross hairs, then the objective for the target, so the image does not move relative to the cross hairs. (Key point: Parallax elimination)

## Related in Surveying I

- [Temporary adjustments of theodolite](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atemporary-adjustments-theodolite) — Surveying I, chapter 7, asked 8×
- [Fundamental lines of theodolite and relations](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afundamental-lines-of-theodolite) — Surveying I, chapter 7, asked 4×
- [Fundamental lines violated; minimising error](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afundamental-lines-error-minimised) — Surveying I, chapter 7