📌 Introduction to Compass Surveying
Compass surveying is one of the earliest and most widely used methods in plane surveying, where a magnetic compass is employed to measure the direction of survey lines with respect to magnetic north. This method is primarily used for mapping small areas, where high precision is not critically required.
In JKSSB exams, especially for posts like Junior Engineer (Civil/Mechanical/Electrical) or Draftsman, compass surveying is a frequently asked topic in both objective and descriptive formats. Understanding it in detail gives you an edge.
🎓 Scope of Compass Surveying in JKSSB Exams
Why JKSSB Aspirants Must Study Compass Surveying:
- Minimum 2–3 questions appear from Compass Surveying in JE (Civil) and Draftsman exams.
- Topics like bearings, local attraction, declination, and included angles are often repeated.
- Simple calculations based on field data are asked in MCQs.
📖 Example from Previous JKSSB JE Paper:
“What is the back bearing of a line having a fore bearing of 215°?”
✅ Correct Answer: 215° – 180° = 35°
🧭 Types of Compasses Used in Surveying
1. Prismatic Compass
- Used widely in India.
- Has a prism attached to the eye vane, enabling simultaneous sighting and reading.
- Readings are taken from 0° to 360°, known as Whole Circle Bearing (WCB).
- The compass needle aligns with magnetic north.
Features:
- Portable and easy to use
- Mounted on a tripod or held in hand
- Graduation increases clockwise
2. Surveyor’s Compass
- Graduated in Quadrantal Bearings (QB): 0° to 90° in each quadrant (NE, SE, SW, NW).
- Used mostly in American systems.
- Reading is taken using a sight vane, and graduation increases in both directions.
Note: Prismatic compass is preferred in competitive exams due to its widespread use in India.
🧭 Types of Bearings in Compass Surveying
Bearings are used to express the direction of a survey line with respect to magnetic north.
1. Whole Circle Bearing (WCB)
- Measured clockwise from magnetic north.
- Range: 0° to 360°
- Common in Prismatic Compass
- Example: North = 0°, East = 90°, South = 180°, West = 270°
2. Quadrantal Bearing (QB)
- Measured from North or South towards East or West
- Range: 0° to 90° in each quadrant
- Example: N30°E, S45°W
🔁 Conversion between WCB and QB
| WCB | QB |
|---|---|
| 45° | N45°E |
| 135° | S45°E |
| 225° | S45°W |
| 315° | N45°W |
📝 Important Tip: These conversions are common in JKSSB MCQs.
📏 Surveying Traverse – Key Concept
A traverse is a series of connected survey lines. The bearings of each line are measured using a compass.
🔄 Types of Traverse:
- Open Traverse: Starts and ends at different points.
- Closed Traverse: Forms a loop, starting and ending at the same point.
JKSSB Tip: Questions often involve computing included angles and checking for closure errors in closed traverse.
🧲 Magnetic Declination and Local Attraction
🌐 Magnetic Declination
- It is the angle between magnetic north and true north.
- Declination can be:
- Easterly (Magnetic North is east of True North)
- Westerly (Magnetic North is west of True North)
- Necessary to apply corrections to measured bearings.
📝 Formula:
True Bearing = Magnetic Bearing ± Declination
⚠️ Local Attraction
Occurs when nearby magnetic materials disturb the compass needle.
How to Detect Local Attraction:
- Measure Fore Bearing (FB) and Back Bearing (BB).
- Normally: FB – BB = 180°
- If not, local attraction is present.
Correction:
- Adjust observed bearings using known unaffected lines as a reference.
📐 Important Surveying Formulas for JKSSB
| Concept | Formula |
|---|---|
| Back Bearing (BB) | FB ± 180° (Add if FB < 180°, Subtract if FB > 180°) |
| Included Angle (Interior) | = FB of next line – BB of previous line |
| Declination Correction | True Bearing = Magnetic Bearing ± Declination |
| Checking Local Attraction | FB – BB = 180° (ideal condition) |
📝 Field Procedure of Compass Surveying
- Select and mark stations in the field.
- Set up the compass over the station using a tripod and plumb bob.
- Level the compass properly using the bubble level.
- Take fore and back bearings for each line.
- Record the readings, ensuring minimal error.
- Check for local attraction and apply corrections.
- Plot the traverse using recorded bearings and lengths.
🎯 Tips for JKSSB Preparation (Compass Surveying)
- Make a chart of bearing conversions (WCB ↔ QB).
- Practice problems on included angles and declination.
- Revise formulas daily.
- Focus on previous year questions – many are repeated.
- Use surveying instruments images to recognize them visually.
🏁 Final Words
Compass Surveying is simple yet scoring if prepared with concept clarity. Understanding types of compasses, types of bearings, and corrections like local attraction and declination will help you solve JKSSB questions with ease.
Stay consistent with revision and practice to master this section.
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