Instruments and Their Setting Up (Compass Survey)
In compass surveying, accurate measurement of directions depends on the proper use and correct setting of instruments. Surveyors use different types of compasses and accessories to determine bearings and carry out fieldwork. Understanding these instruments and their correct setup is essential for students of the Surveyor trade.
Instruments Used in Compass Survey
The main instruments used in compass surveying are as follows:
1. Prismatic Compass
The prismatic compass is the most commonly used instrument in compass surveying. It is used to measure bearings in the Whole Circle Bearing (WCB) system. It consists of a magnetic needle, graduated circle, prism, object vane, and sight vane.
- Measures bearings directly
- Provides accurate readings
- Can be used with tripod or handheld
2. Surveyor’s Compass
The surveyor’s compass is another type of compass used to measure bearings in the Quadrantal Bearing (QB) system. It is less precise compared to the prismatic compass.
- Simple to use
- Suitable for rough surveys
3. Tripod Stand
A tripod is used to support the compass. It ensures stability and proper leveling of the instrument.
- Adjustable height
- Provides firm support
4. Ranging Rod
Ranging rods are used to mark survey stations and help in aligning survey lines.
5. Pegs
Pegs are used to mark fixed points or stations on the ground.
6. Chain or Tape
Used for measuring distances between survey stations.
Setting Up of Compass
Proper setting up of the compass is necessary to obtain accurate readings. The following steps should be followed:
1. Centering
Centering means placing the compass exactly over the survey station. This is done using a plumb bob or by adjusting the tripod legs.
2. Levelling
Levelling ensures that the compass is placed horizontally. The tripod legs are adjusted until the instrument is level. Proper levelling allows the magnetic needle to move freely.
3. Focusing the Prism
In a prismatic compass, the prism is adjusted so that the readings on the graduated circle are clearly visible.
4. Adjustment of Sight Vanes
The object vane and sight vane are aligned properly to sight the target point accurately.
5. Free Movement of Magnetic Needle
Ensure that the magnetic needle moves freely and is not obstructed. It should come to rest quickly and point towards the magnetic north.
6. Taking the Reading
After proper setup, the surveyor sights the object and reads the bearing through the prism. The reading should be taken carefully to avoid errors.
Precautions While Setting Up
- Keep the compass away from magnetic materials
- Ensure proper centering and levelling
- Check that the needle moves freely
- Take readings carefully and avoid parallax error
- Protect the instrument from dust and damage
Common Errors During Setup
- Improper centering of the instrument
- Incorrect levelling
- Local attraction affecting readings
- Faulty adjustment of prism or sight vane
Importance in Surveyor Trade
Proper knowledge of instruments and their setup is essential for accurate compass surveying. Incorrect setup can lead to errors in bearing measurement and affect the entire survey work.
Students are trained to handle instruments carefully and follow correct procedures to ensure reliable results.
Conclusion
Instruments such as prismatic compass, tripod, and accessories play an important role in compass surveying. Proper setting up of these instruments ensures accurate measurement of directions.
For ITI Surveyor trainees, mastering the use and setup of instruments is a fundamental skill that forms the basis for advanced surveying techniques.