Surveying is a critical component of many engineering and construction projects, requiring precise measurements of angles and directions. Compass surveying, which uses a magnetic compass, is a fundamental method for determining angles and survey line directions. This article delves into the principles, features, and types of magnetic compasses used in surveying, as well as the necessary adjustments for their accurate operation.
Principle of the Magnetic Compass
A magnetic compass operates on the principle of magnetization and alignment with Earth’s magnetic field. When a narrow strip of steel or iron is magnetized and suspended about its center, it oscillates freely around a vertical axis and aligns itself along the magnetic meridian at the point of observation. This alignment is used to determine magnetic bearings of survey lines.
Bearings are measured using either the Whole Bearing System (WBS) or the Quadrantal Bearing System (QBS), depending on the compass type:
- Whole Bearing System (WBS): Bearings are measured clockwise from 0° to 360°.
- Quadrantal Bearing System (QBS): Bearings are measured within quadrants, with directions given as North or South followed by an angular measurement toward East or West.
Features of a Magnetic Compass
Magnetic compasses used in surveying have several essential components:
- Magnetic Needle: Aligns with the Earth’s magnetic field to establish the magnetic meridian.
- Line of Sight: Used to visually align with the endpoint of the survey line.
- Graduated Circle: Reads the directions of lines, either fixed to the compass box or the needle itself.
- Compass Box: Houses the components and provides stability. It is supported by a tripod or stand.
These features ensure the compass is reliable and accurate for field measurements.
Types of Magnetic Compasses Used in Surveying
1. Prismatic Compass
The prismatic compass is a portable and versatile magnetic compass often used in field surveys. It can be handheld or mounted on a tripod.
- Components:
- The line of sight is defined by the object vane and eye vane.
- A prism is integrated to allow simultaneous sighting and reading.
- System of Graduation:
- Bearings increase clockwise:
- South (0°)
- West (90°)
- North (180°)
- East (270°)
- Bearings increase clockwise:
- Usage: The compass is centered over the starting station, leveled, and adjusted to align with the magnetic meridian. Bearings are then measured by sighting the next station.
- Temporary Adjustments:
- Centering: Ensuring the instrument is positioned exactly over the survey station, typically done by adjusting the tripod or using a plumb-bob.
- Levelling: Ensuring the graduated disc swings freely and remains horizontal. This can be achieved with a tripod or by hand.
- Focusing the Prism: Sliding the prism attachment until the readings are sharp and clear.
2. Surveyor’s Compass
The surveyor’s compass is another commonly used magnetic compass, consisting of a circular brass box housing a magnetic needle. The needle swings over a graduated circle marked in degrees (360°).
- Components:
- A pair of sights are located along the north-south axis for horizontal angle measurement.
- The compass is typically mounted on a tripod with a ball and socket mechanism for leveling.
- System of Graduation: The compass uses a full 360° graduated circle for bearing measurements.
- Temporary Adjustments: Similar to the prismatic compass, these adjustments ensure proper positioning, leveling, and focus.
- Permanent Adjustments:
- Adjustment of Levels: Ensuring the base is perfectly horizontal.
- Adjustment of Needle: Verifying that the magnetic needle aligns correctly with the compass circle.
- Adjustment of Sight Vanes: Ensuring the vanes are aligned and properly fixed.
- Adjustment of Pivot: Guaranteeing the needle swings freely and accurately over the circle.
Conclusion
Magnetic compasses, including the prismatic and surveyor’s compasses, are invaluable tools in surveying for determining directions and angles. Their functionality is rooted in the principles of magnetization and alignment with the Earth’s magnetic field. Proper use of these instruments requires an understanding of their components, systems of graduation, and the necessary adjustments. Whether for small-scale surveys or preliminary site investigations, magnetic compasses remain a practical and efficient choice for fieldwork.