Why this topic is important for ECET?
In ECET 2026 Civil, Surveying contributes a significant weightage. Among surveying methods, Compass Surveying is crucial because it deals with quick and large-scale surveys using a magnetic compass. Understanding types of bearings, methods of traversing, and sources of errors helps you solve both theory and numerical problems. Errors in compass readings are a common MCQ area, so mastering this topic ensures easy scoring.
📘 Concept Notes
1. Compass Survey Basics
- Definition: Compass survey is a method where directions of survey lines are measured using a magnetic compass and lengths using a chain/tape.
- Instruments used: Prismatic Compass, Surveyor’s Compass.
2. Types of Bearings
- Whole Circle Bearing (WCB): Measured clockwise from North (0° to 360°).
- Reduced Bearing (RB) or Quadrantal Bearing (QB): Measured from North or South towards East or West (0°–90°).
Conversion:
- From WCB to RB:
- If WCB between 0°–90° → NE
- 90°–180° → SE
- 180°–270° → SW
- 270°–360° → NW
3. Fore Bearing (FB) & Back Bearing (BB)
- Fore Bearing (FB): Bearing measured in the direction of survey line.
- Back Bearing (BB): Bearing measured in the opposite direction.
- Relationship:
4. Types of Compass Errors
- Instrumental Errors:
- Improper adjustment of sighting vane, graduation errors, imperfect needle.
- Personal Errors:
- Inaccurate centering, wrong reading, careless sighting.
- Natural Errors:
- Local attraction due to magnetic materials, variation of magnetic declination, proximity of electric wires.
5. Local Attraction
- When magnetic needle is disturbed by local magnetic sources (iron, steel, wires, etc.).
- Detected if:
- Difference between FB and BB is not exactly 180°.
6. Correction for Declination
- Magnetic Declination (δ): Angle between magnetic meridian and true meridian.
- True Bearing (TB):
7. Traverse Computations
- Closed traverse check:
Sum of FB – Sum of BB = 0 (if free from local attraction).
⚙️ Formulas
🔟 10 MCQs
Q1. In whole circle bearing system, bearings are measured from:
a) South clockwise
b) North clockwise
c) South anticlockwise
d) North anticlockwise
Q2. If FB of a line is 75°, BB = ?
a) 75°
b) 255°
c) 285°
d) 105°
Q3. Reduced Bearing of 210° (WCB) is:
a) N 30° W
b) S 30° W
c) S 60° E
d) N 60° E
Q4. If FB = 45° and BB = 226°, what is indicated?
a) Correct observation
b) Local attraction
c) Declination
d) Instrumental error only
Q5. Prismatic compass reads:
a) WCB directly
b) RB directly
c) Both RB and WCB
d) Declination only
Q6. If the magnetic declination is 2°E and magnetic bearing is 100°, true bearing = ?
a) 98°
b) 100°
c) 102°
d) 120°
Q7. In compass surveying, length of line is measured using:
a) Compass only
b) Theodolite
c) Chain/tape
d) GPS
Q8. Local attraction is detected if:
a) FB – BB = 0°
b) FB – BB ≠ 180°
c) WCB = RB
d) None
Q9. Which is NOT a natural error?
a) Magnetic declination
b) Local attraction
c) Graduation error
d) Proximity of electric cables
Q10. If WCB of line = 300°, RB = ?
a) N 60° W
b) S 60° W
c) N 60° E
d) S 30° W
✅ Answer Key
| Q | Answer |
|---|---|
| 1 | b |
| 2 | b |
| 3 | b |
| 4 | b |
| 5 | a |
| 6 | c |
| 7 | c |
| 8 | b |
| 9 | c |
| 10 | a |
🧠 Explanations
- Q1: WCB measured clockwise from North → (b).
- Q2: BB = FB + 180 = 75 + 180 = 255° → (b).
- Q3: 210° lies in 180°–270° quadrant → S (210–180 = 30°) W → (b).
- Q4: Normally FB–BB = 180°. Here = 181°, showing local attraction → (b).
- Q5: Prismatic compass reads WCB directly → (a).
- Q6: TB = MB + Declination = 100° + 2°E = 102° → (c).
- Q7: Lengths measured by chain/tape → (c).
- Q8: Local attraction when FB–BB ≠ 180° → (b).
- Q9: Graduation error = instrumental error, not natural → (c).
- Q10: 300° lies in 270°–360° quadrant → N (360–300 = 60°) W → (a).
🎯 Motivation / Why Practice Matters
In ECET 2026, Surveying problems are fast-scorable if you know conversions between bearings and error checks.
- Compass survey questions are formula-based → low chance of error if you practice.
- Common trap: mixing WCB and RB → practice ensures accuracy.
👉 Mastering this topic gives you an edge in speed and correctness compared to other students.
📲 CTA
Join our WHATSAPP group for ECET 2026 updates and discussions →
👉 https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9
