---
title: "Optical-reading repeating and directional theodolites"
subject: "Surveying I"
chapter: 7
key: "semester_3/surveying_i:optical-repeating-and-directional-theodolite"
source: https://www.bctengineer.com/q/semester_3/surveying_i/optical-repeating-and-directional-theodolite
site: BCT Engineering
---

# Optical-reading repeating and directional theodolites

Surveying I · Chapter 7: Transit and Theodolite

- Asked in: 2058 Chaitra, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/optical-repeating-and-directional-theodolite
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-7

## Question

Explain (i) the optical-reading repeating theodolite and (ii) the optical-reading directional theodolite.

## Short answer

The optical repearing theodolite has upper and lower plates allowing angle repetition, read via an optical micrometer. The optical directional theodolite has a fixed horizontal circle and one alidade motion for direction measurement by reiteration, offering higher precision and used in triangulation.

## Answer

In an **optical-reading** theodolite the glass circles are read through a microscope; there are no open verniers. Two types are used.

#### (i) Optical-reading repeating theodolite
- It has an **upper plate** and a **lower plate**, each with its own clamp and tangent screw, as in a vernier repeating theodolite.
- The graduated circle can be clamped to the alidade (upper part) or to the lower part. So an angle can be **repeated** (added) several times on the circle without resetting the reading.
- The circle is read through a micrometer or optical-scale microscope; both sides of the circle are brought together in the field of view (reducing eccentricity).
- It is used for the repetition method as well as the ordinary method.

#### (ii) Optical-reading directional theodolite
- The horizontal circle is fixed relative to the base during observation; it has only **one clamp and tangent screw** for the alidade (no separate lower motion), so repetition is not possible.
- Directions to each station are read from the fixed circle, and angles are found from differences of directions. This is the **direction (reiteration) method**.
- The circle is turned by a **circle-setting screw** to change the zero between sets.
- It gives more accurate results (reading to 1 second or 0.1 second) and it is quick, so it is used in triangulation.

| Feature | Repeating | Directional |
|---|---|---|
| Motions | Upper and lower | One only |
| Repetition | Possible | Not possible |
| Method | Repetition and ordinary | Direction (reiteration) |
| Typical use | Traverse, ordinary work | Triangulation, precise work |

## Check yourself

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

1. What lets an optical repeating theodolite repeat an angle?
   A. A circle-setting screw only
   B. Upper and lower plates with separate clamps; the circle can be clamped to either part
   C. A fixed circle with one clamp
   D. Two microscopes
   Answer: B. The circle can be clamped to the alidade or the lower part, so the angle can be added several times without resetting. (Key point: Repeating theodolite feature)

2. Why is repetition not possible in a directional theodolite?
   A. Its circle is fixed during observation and it has only one clamp and tangent screw for the alidade
   B. It has no telescope
   C. It reads only vertical angles
   D. It has verniers instead of a microscope
   Answer: A. With a fixed circle and no separate lower motion, the angle cannot be accumulated on the circle. (Key point: Directional theodolite)

3. Which method and use suit the directional theodolite?
   A. Repetition method for ordinary traverse
   B. Direction (reiteration) method for triangulation and precise work
   C. Ordinary method for levelling
   D. Tangent method for plotting
   Answer: B. It reads directions from a fixed circle and is accurate and quick, so is used in triangulation. (Key point: Use of directional theodolite)

## 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×
- [Theodolite construction principle; micrometer](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atheodolite-construction-principle-micrometer) — Surveying I, chapter 7, asked 2×
- [Mean angles and VCR, station O (reading 179°59′40″)](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Amean-direction-o-abcd-first-reversed-179-59-40) — Surveying I, chapter 7, asked 3×

## Related in other subjects

- [Principles and planning of theodolite traverse](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atheodolite-traverse-principles-planning) — Surveying II, chapter 1
- [Optical lever comparator magnification](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Aoptical-lever-comparator-numerical) — Metrology, chapter 7
- [Advantages of optical fiber over copper](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aoptical-fiber-advantages) — Instrumentation II, chapter 5, asked 3×
- [Optical fiber advantages and disadvantages](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aoptical-fiber-pros-cons) — Instrumentation II, chapter 5