Uses of Chain/Tape, Testing of a Chain and Corrections

In surveying, measurement of distances is one of the most fundamental operations. Chains and tapes are commonly used instruments for this purpose. Despite the development of modern electronic instruments, chains and tapes are still widely used in basic surveying work due to their simplicity and reliability. Understanding their uses, methods of testing, and necessary corrections is essential for students in the Surveyor trade.

Uses of Chain and Tape

Chains and tapes are used for direct measurement of horizontal distances in the field. They are basic tools in chain surveying and are also used in other surveying methods for checking and setting out measurements.

1. Measurement of Distances

The primary use of a chain or tape is to measure linear distances between two points on the ground. This is essential for preparing maps and plans.

2. Setting Out Work

Chains and tapes are used to set out distances on the ground according to the design. This is important in construction projects such as buildings, roads, and layouts.

3. Offsets Measurement

They are used to measure offsets, which are the lateral distances from the survey line to details such as buildings, trees, or boundaries.

4. Checking Measurements

Chains and tapes are used to verify measurements taken by other instruments, ensuring accuracy and reliability.

5. Plotting and Mapping

Measured distances are used to prepare maps and drawings. Accurate measurement ensures correct representation of the surveyed area.

Types of Chains and Tapes

Different types of chains and tapes are used in surveying:

• Metric Chain: Commonly 20 m or 30 m long, divided into links.
• Steel Tape: Made of steel, used for accurate measurements.
• Cloth or Linen Tape: Used for rough measurements.
• Fiberglass Tape: Resistant to moisture and corrosion.

Testing of a Chain

Over time, chains may become longer or shorter due to wear and tear, bending of links, or improper handling. Therefore, it is necessary to test the chain periodically to ensure its accuracy.

Methods of Testing

The standard method of testing a chain is by comparing it with a standard length on a test gauge or a known measured distance.

• Lay the chain on a flat surface
• Compare it with a standard length (e.g., 20 m or 30 m)
• Check for any difference in length

If the chain is found to be longer or shorter than the standard length, corrections must be applied.

Errors in Chain Measurement

Errors in chain measurement can occur due to various reasons:

• Wear and tear of links
• Bending or twisting of chain
• Temperature changes
• Incorrect handling
• Sagging of chain during measurement

Corrections in Chain Measurement

To obtain accurate results, necessary corrections must be applied to the measured distances.

1. Correction for Length

If the chain is longer than the standard length, the measured distance will be less than the actual distance. In this case, a positive correction is applied.

If the chain is shorter, the measured distance will be more than the actual distance, and a negative correction is applied.

2. Correction for Temperature

Chains expand in high temperatures and contract in low temperatures. Temperature correction is applied to account for this change.

3. Correction for Sag

When a chain is suspended between two points, it sags under its own weight. This increases the measured length, so a negative correction is applied.

4. Correction for Slope

If the ground is sloping, the measured distance along the slope is greater than the horizontal distance. Correction is applied to obtain the true horizontal distance.

5. Correction for Pull

Variation in the applied pull or tension can affect the length of the chain. Proper tension should be maintained during measurement.

Precautions While Using Chain/Tape

• Ensure the chain is properly tested before use
• Avoid dragging the chain on rough surfaces
• Keep the chain clean and free from rust
• Apply proper tension during measurement
• Use ranging rods for alignment
• Avoid measurement in extreme weather conditions

Importance in Surveyor Trade

Chains and tapes are fundamental tools in surveying. Even with the availability of advanced instruments, they are still used for basic measurements and verification.

Understanding their proper use, testing, and corrections helps in achieving accurate results. This knowledge is essential for surveyor trainees and forms the foundation for advanced surveying techniques.

Conclusion

Chains and tapes are simple yet important instruments used in surveying for measuring distances. Regular testing and application of necessary corrections are essential to maintain accuracy.

For students in the Surveyor trade, mastering the use of chain and tape is a basic but crucial step in their training. With proper handling and understanding of corrections, they can perform measurements accurately and efficiently.