What is Compass Surveying?
Definition:
Compass surveying is a type of plane surveying in which the bearings (directions) of survey lines are measured with a magnetic compass and the lengths of those lines are measured using a tape or chain.
Key Points:
Directions are measured with respect to a magnetic meridian.
It is used for large and uneven areas where triangulation is difficult.
The method is quick, simple, and economical.
🔹 Traverse in Compass Surveying
A Traverse is a series of connected survey lines whose lengths and bearings are measured.
When all the survey stations are connected by straight lines, the framework formed is called a Traverse.
Types of Traverse:
Type Description Example
Closed Traverse Starts and ends at the same point Survey of lakes, towns, forests, fields
Open Traverse Starts and ends at different points Survey of roads, railways, canals, tunnels
Note:
Closed traverses are useful for boundary surveys.
Open traverses are used for long narrow strips like roads and pipelines.
🔹 Instruments Used for Measuring Angles
Magnetic Compass
Box Sextant
Theodolite
Among these, the magnetic compass is the simplest and most commonly used instrument.
🔹 Types of Compass
There are mainly three types of compasses used in surveying:
🧭 Surveyor’s Compass
Graduations increase clockwise from South (0°) to South (360°).
Readings are taken in Quadrantal Bearing (QB) system.
🔍 Prismatic Compass
Graduations increase clockwise from North (0°) to North (360°).
Readings are taken in Whole Circle Bearing (WCB) system.
Equipped with a prism for magnified readings.
Can be used handheld or mounted on a tripod.
📏 Trough Compass
Used in mining or tunnel surveying for short lines.
Simple rectangular box with a magnetic needle.
🔹 Parts of a Compass
Part Description
1. Magnetic Needle Magnetized steel needle that aligns with the magnetic north.
2. Pivot Sharp steel pin supporting the needle for free rotation.
3. Graduated Ring Circular ring graduated from 0°–360° (in Prismatic Compass).
4. Eye Vane Vertical slit used to sight the object; holds the prism.
5. Object Vane Vertical wire or hair used for sighting.
6. Line of Sight Imaginary line joining the eye vane, object vane, and target.
7. Prism Magnifies the reading on the graduated ring.
8. Lifting Pin & Lever Lifts the needle to avoid wear when not in use.
9. Brake Pin Holds the graduated ring steady during reading.
10. Compass Box Circular brass box (100–150 mm) enclosing all parts.
11. Tripod (Jacob’s Staff) Wooden stand used to hold and level the compass.
🔹 Magnetic Dip (Inclination of Needle)
The magnetic needle never remains perfectly horizontal.
It dips downward due to the earth’s magnetic field.
In the Northern Hemisphere, the north end dips downward.
In the Southern Hemisphere, the south end dips downward.
At the Equator, the needle remains horizontal.
The vertical angle between the magnetic needle and the horizontal plane is called the Magnetic Dip or Inclination.
🔹 Advantages of Compass Surveying
Quick and simple in operation
Portable and lightweight instrument
Suitable for rough or hilly areas
Cost-effective for small surveys
🔹 Limitations
Affected by local attraction (magnetic materials nearby)
Accuracy is less compared to theodolite surveying
Not suitable for precise large-scale work
🏁 Conclusion
Compass Surveying is a basic yet powerful technique in civil engineering to determine directions and angles using a magnetic compass.
It is still widely used in preliminary surveys, topographical studies, and route surveys due to its simplicity and speed.
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