Chain Survey – Instruments Employed
In chain surveying, various instruments are used to measure distances, mark points, and take offsets in the field. These instruments are simple, easy to use, and essential for accurate measurement. In ITI Draughtsman Civil, understanding these tools is important for conducting field surveys and preparing accurate drawings.
1. Chain
The chain is the main instrument used for measuring distances in chain surveying. It is made of steel links connected together.
- Common length: 20 meters or 30 meters.
- Each link is usually 20 cm long.
- Provided with handles at both ends.
- Types include metric chain, Gunter’s chain, and engineer’s chain.
2. Tape
A tape is used for more accurate measurement than a chain. It is flexible and easy to handle.
- Made of steel, linen, or fiberglass.
- Available in different lengths (10 m, 20 m, 30 m).
- Used for precise measurements and short distances.
3. Arrows (Chain Pins)
Arrows are used to mark the end of each chain length during measurement.
- Made of steel wire.
- Usually 10 arrows are provided with a chain.
- Helps in counting the number of chain lengths measured.
4. Ranging Rods
Ranging rods are used to mark stations and to align straight lines during surveying.
- Usually 2 to 3 meters long.
- Painted in alternate red and white bands for visibility.
- Fitted with a pointed iron shoe at the bottom.
5. Offset Rod
Offset rods are used for measuring short offsets and setting right angles from the survey line.
- Provided with a hook or notch at the top.
- Usually 3 meters long.
6. Cross Staff
A cross staff is used for setting out right angles in the field.
- Consists of a head with four slots at right angles.
- Mounted on a staff.
- Used for taking perpendicular offsets.
7. Optical Square
An optical square is a more accurate instrument than a cross staff for setting right angles.
- Uses mirrors or prisms.
- Provides precise perpendicular alignment.
8. Pegs
Pegs are used to mark survey stations permanently on the ground.
- Made of wood.
- Driven into the ground using a hammer.
- Top is usually marked with a nail or paint.
9. Plumb Bob
A plumb bob is used to transfer points vertically to the ground.
- Consists of a pointed weight suspended by a string.
- Ensures vertical alignment.
10. Field Book
The field book is used to record all measurements and observations taken during the survey.
- Contains sketches and notes.
- Essential for preparing final drawings.
Importance of Instruments
Proper use of these instruments ensures accuracy and efficiency in surveying work. Each tool has a specific purpose, and correct handling is necessary to avoid errors.
Role of Draughtsman Civil
A Draughtsman Civil uses the data collected using these instruments to prepare detailed plans and drawings. Understanding these tools helps in interpreting field data correctly.
Conclusion
Instruments used in chain surveying are simple but essential for accurate measurement. Tools like chains, tapes, ranging rods, and cross staff help in carrying out survey work efficiently.
For ITI Draughtsman Civil students, knowledge of these instruments is fundamental for learning surveying techniques and preparing precise construction drawings.