Basic Terms Used in Compass Survey
Compass surveying is one of the methods used in surveying to measure the direction of survey lines with the help of a compass. It is widely used for small areas where high precision is not required. Understanding the basic terms used in compass survey is essential for Surveyor trade trainees, as these terms form the foundation of direction measurement and fieldwork.
Compass Survey
Compass surveying involves the measurement of angles in the form of bearings using a magnetic compass. The direction of survey lines is determined with respect to the magnetic meridian.
Meridian
A meridian is a reference line used to measure directions. It is a fixed line passing through a point. There are different types of meridians:
- True Meridian: The line passing through the geographical north and south poles.
- Magnetic Meridian: The direction indicated by the magnetic needle.
- Arbitrary Meridian: Any convenient assumed direction.
Bearing
Bearing is the horizontal angle between a reference meridian and a survey line. It indicates the direction of a line.
Types of Bearings
- Whole Circle Bearing (WCB): Measured clockwise from north (0° to 360°).
- Quadrantal Bearing (QB): Measured from north or south towards east or west (0° to 90°).
Fore Bearing (F.B.)
Fore bearing is the bearing of a line measured in the forward direction of survey (from starting point to next point).
Back Bearing (B.B.)
Back bearing is the bearing of the same line measured in the opposite direction.
Relation:
- In WCB: B.B. = F.B. ± 180°
- In QB: Direction letters are reversed
Magnetic Declination
Magnetic declination is the angle between the true meridian and the magnetic meridian. It may be east or west depending on the location.
Magnetic Dip
Magnetic dip is the angle made by the magnetic needle with the horizontal plane. It varies from place to place.
Local Attraction
Local attraction is the disturbance in the magnetic needle caused by nearby magnetic objects such as iron, electric lines, or machinery. It affects the accuracy of readings.
Included Angle
The angle between two survey lines at a station is known as the included angle. It is used to determine the shape of the traverse.
Traverse
A traverse is a series of connected lines whose lengths and directions are measured. It forms the framework of compass surveying.
- Open Traverse: Does not return to the starting point.
- Closed Traverse: Forms a closed figure.
Prismatic Compass
A prismatic compass is a commonly used instrument in compass surveying. It measures bearings in the whole circle system and provides direct readings.
Survey Line
A survey line is a line connecting two survey stations. Its length and direction are measured during surveying.
Station
A station is a fixed point on the ground where observations are taken.
Importance of These Terms
- Helps in understanding direction measurement
- Ensures accurate fieldwork
- Useful in plotting and calculations
- Forms the base for advanced surveying methods
Conclusion
Compass surveying is an important method used for measuring directions in the field. Understanding the basic terms such as bearing, meridian, declination, and local attraction is essential for accurate surveying work. For ITI Surveyor trainees, these concepts provide a strong foundation for practical and theoretical knowledge in surveying.