---
title: "Prismatic compass versus surveyor's compass"
subject: "Surveying I"
chapter: 4
key: "semester_3/surveying_i:prismatic-vs-surveyors-compass"
source: https://www.bctengineer.com/q/semester_3/surveying_i/prismatic-vs-surveyors-compass
site: BCT Engineering
---

# Prismatic compass versus surveyor's compass

Surveying I · Chapter 4: The Compass

- Asked in: 2080 Bhadra, 4 marks; 2066 Jestha, 8 marks (asked 2 times)
- Video: [Prismatic Compass vs Surveyor Compass | Surveying | Civil Engineering | By Abhishek Sir Unacademy](https://www.youtube.com/watch?v=w1ENWk7x49Y) — Civil Yodha by Unacademy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/prismatic-vs-surveyors-compass
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-4

## Question

What are the differences between a prismatic compass and a surveyor's compass?

## Short answer

Prismatic compass: graduated ring fixed to needle, whole circle bearings (0–360°) read through prism while sighting; fast, hand-held. Surveyor's compass: fixed ring, quadrantal bearings (0–90°) read directly; needs tripod, separate sighting and reading.

## Answer

Both compasses measure the magnetic bearing of a line with a magnetic needle, but they differ in the reading system, graduation and sighting arrangement.

| Point | Prismatic compass | Surveyor's compass |
|---|---|---|
| Graduation | Graduated ring (card) fixed to the needle, $0^\circ$–$360^\circ$ (whole circle bearing) | Ring fixed to the box; graduated in quadrants, $0^\circ$–$90^\circ$ (quadrantal bearing) |
| Needle | Needle is attached to the ring; the ring swings with the needle | Needle swings freely over the ring; the ring is fixed to the box |
| Reading | Taken through a prism at the eye slit, so the reading is seen while sighting | Taken by looking down directly on the ring; cannot be read while sighting |
| Sighting | Object vane with a hair and eye slit with a prism; sighting and reading at the same time | Sight vanes (slit and hair); the eye must be moved between sighting and reading |
| Tripod | Can be used with or without a tripod (hand-held) | Needs a tripod |
| Graduation figures | Engraved upside down (inverted), so they appear erect when read through the prism | Engraved erect and read directly; E and W are interchanged on the ring |
| Reading accuracy | Estimated to about 30′ | About 15′ to 30′ |
| Speed and use | Quick, portable, widely used in rough work | Slower and used less now |

**Main advantage of the prismatic compass:** sighting and reading are done together, so there is less chance of disturbing the instrument and the survey is faster.

## Check yourself

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

1. The prismatic compass is graduated in:
   A. Quadrantal 0 to 90 degrees
   B. Reduced bearing only
   C. Whole circle bearing 0 to 360 degrees
   D. Radians
   Answer: C. The surveyor's compass uses quadrants. (Key point: Graduation)

2. In the prismatic compass the ring:
   A. Is fixed to the box
   B. Is absent
   C. Is horizontal glass
   D. Is fixed to the needle and swings with it
   Answer: D. In the surveyor's compass the ring is fixed to the box. (Key point: Ring and needle)

3. Reading in a prismatic compass is made:
   A. Through a prism while sighting
   B. By looking down, after moving the eye
   C. From a vernier on a telescope
   D. On a digital display
   Answer: A. This allows sighting and reading simultaneously. (Key point: Reading)

4. The prismatic compass:
   A. Always needs a tripod
   B. Can be used with or without a tripod
   C. Cannot be tripod-mounted
   D. Needs two tripods
   Answer: B. The surveyor's compass requires a tripod. (Key point: Tripod)

## Related in Surveying I

- [Corrected bearings of closed traverse PQRST](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aclosed-traverse-pqrst-correct-bearings) — Surveying I, chapter 4

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