Chain surveying, a fundamental method in civil engineering, relies exclusively on linear measurements to map terrains, especially for small and relatively open areas. The accuracy and efficiency of this method are highly dependent on the quality and proper use of specific surveying instruments. This infographic provides a detailed overview of the key equipment utilized in chain surveying, explaining their purpose and practical application in the field.
Detailed Overview of Chain Surveying Equipment
Each piece of equipment plays a distinct role in ensuring the precision and systematic execution of linear measurements, from marking points to establishing perpendicular lines.
| Equipment | Description | Purpose in Chain Surveying |
|---|---|---|
| Pegs / Wood Stakes | Small wooden or metallic stakes, typically 25-30 cm long, pointed at one end and flat at the top for hammering. | Used for marking and establishing the position of survey stations, intersections of lines, or other critical points on the ground. They serve as semi-permanent reference points. |
| Arrows | Steel pins, usually 25cm to 50cm long, with a pointed end to be driven into the ground and a circular or elliptical ring at the top for easy handling. | Used for marking the end of each chain or tape length during linear measurements. The leader inserts an arrow at the end of each measurement, and the follower collects them. |
| Chains (Surveyor’s & Gunter’s) | Composed of galvanized mild steel links joined by rings. Common lengths are 20m or 30m. Gunter’s chain is 66 feet long. | Primary instrument for direct linear distance measurement, especially where high precision of tape is not strictly required. Used for basic framework measurements. |
| Tapes (Steel, Linen, Invar) | Flexible measuring bands made of various materials. Steel tapes offer good accuracy, linen tapes are for rough work, and invar tapes provide highest precision (low thermal expansion). | Used for more accurate linear measurements than chains, especially for offsets, short distances, or setting out. Invar tapes are used for base lines. |
| Ranging Rods | Wooden or metallic poles, usually 2m or 3m long, painted in alternating bands of red and white or black and white (20cm each). | Used for marking the positions of stations and for ranging (aligning intermediate points) along a survey line to ensure measurements are taken along a straight path. |
| Offset Rods | Similar to ranging rods but often with a fork or hook at one end. They may also have graduations. | Used for measuring short offsets perpendicular or oblique to the chain line, and for holding the tape taut during offset measurements. |
| Plumb Bob | A pointed metallic weight suspended from a string. | Used for transferring points to the ground or from ground to instrument, ensuring vertical alignment (e.g., when measuring on slopes by stepping). Essential for accurate vertical projection. |
| Cross-Staff (Open, French, Adjustable) | Simple instrument consisting of a frame with two pairs of vertical slits or octagonal/circular frames, used to set out right angles. | Used for establishing perpendicular offsets from the chain line quickly in the field without angular instruments. |
| Optical Square | A hand instrument based on the principle of double reflection, capable of setting out a line exactly perpendicular to a chain line. More accurate than a cross-staff. | Precisely establishing perpendicular offsets and checking right angles for boundary lines. |
| Prism Square | Similar to an optical square but uses prisms instead of mirrors for reflection, making it more robust and easier to use. | Functions identically to the optical square for setting out perpendicular lines with high accuracy. |
| Clinometer | A hand instrument used for measuring the angle of slope or elevation/depression of a ground surface. | Used to determine the slope for applying slope correction to measured distances on uneven terrain. |
Importance of Proper Equipment in Surveying
The reliability of survey data directly correlates with the quality and appropriate selection of equipment. Proper instruments, coupled with skilled handling, are critical for achieving desired accuracy and efficiency.
- Accuracy & Precision: High-quality, well-calibrated instruments ensure that measurements are as close to the true value as possible, minimizing errors.
- Efficiency: Correct equipment streamlines fieldwork, reducing the time and effort required to complete a survey.
- Safety: Properly functioning equipment contributes to a safer working environment for the survey team.
- Data Reliability: Reliable equipment leads to dependable data, which is crucial for sound engineering design and construction.
- Cost-Effectiveness: While initial investment might be higher, quality equipment reduces re-work, saving significant project costs in the long run.
GATE Exam Practice Questions & Explanations on Equipment
Test your understanding of the equipment used in chain surveying with these practice questions, commonly encountered in civil engineering competitive exams like GATE.
1. Which of the following instruments is primarily used for direct linear measurement where high accuracy is not strictly required?
Answer: Chain
Chains are suitable for basic linear measurements in chain surveying, though tapes offer higher precision for more detailed work.
2. Steel pins, typically 25-50 cm long, used to mark the end of each chain or tape length during linear measurements, are called:
Answer: Arrows
Arrows are specifically designed for this purpose, aiding in accurate count of chain lengths and precise marking of points.
3. Which type of tape provides the highest precision due to its very low thermal expansion?
Answer: Invar Tape
Invar tapes are made of a nickel-steel alloy with minimal thermal expansion, making them ideal for precise baseline measurements.
4. Ranging rods are primarily used for:
Answer: Marking the positions of stations and aligning intermediate points along a survey line.
They ensure that measurements are taken along a straight line between two points, a process known as ranging.
5. For establishing perpendicular offsets from a chain line quickly in the field without an angular instrument, which tool is commonly used?
Answer: Cross-Staff or Optical Square
Both instruments are designed to set out right angles directly on the field, with the Optical Square offering higher precision.
6. A pointed metallic weight suspended from a string, used for transferring points to the ground or ensuring vertical alignment, is a:
Answer: Plumb Bob
The plumb bob is essential for accurate vertical projection, particularly when measurements involve changes in elevation or transferring points from instrument to ground.
7. What is the main characteristic of a Gunter’s chain?
Answer: It is 66 feet long.
Gunter’s chain, at 66 feet, is historically significant for land measurement, where 10 chains equal 1 furlong and 80 chains equal 1 mile.
8. Wooden or metallic stakes driven into the ground to mark survey stations are generally referred to as:
Answer: Pegs or Wood Stakes
Pegs provide fixed, identifiable points on the ground for the duration of the survey.
9. Which instrument uses prisms for reflection and is known for being more robust than a mirror-based optical square?
Answer: Prism Square
The prism square uses prisms, which are less prone to damage or misalignment compared to mirrors, making it a durable choice for setting out perpendiculars.
10. What is the primary function of an offset rod?
Answer: For measuring short offsets and holding the tape taut during offset measurements.
Offset rods assist in measuring lateral distances from the main survey line to features, often perpendicular to the line.
11. A Clinometer is used in surveying to measure:
Answer: Angle of slope or elevation/depression of a ground surface.
The clinometer helps in determining the inclination of the ground, which is essential for applying slope corrections to linear measurements.
12. For achieving the highest accuracy in linear measurement in chain surveying, which type of tape should be preferred for measuring the baseline?
Answer: Invar Tape
Invar tapes are chosen for baselines due to their very low coefficient of thermal expansion, which minimizes errors caused by temperature variations.
13. The main difference between a ranging rod and an offset rod lies in:
Answer: The presence of a fork or hook and graduations on an offset rod.
While both are rods, offset rods are often designed with features to assist specifically in measuring and holding tapes for offsets.
14. When using a chain, how are the chain lengths typically marked on the ground by the leader?
Answer: By inserting arrows into the ground.
This is the standard procedure to ensure precise marking of each chain length during the chaining process.
15. Which of the following is NOT a primary instrument for direct linear measurement in chain surveying?
Answer: Theodolite
A theodolite is an angular measuring instrument. While it can be used for indirect linear measurement (e.g., tacheometry), it is not used for direct chaining or taping.
16. The alternating colored bands on a ranging rod (e.g., red and white) are usually of what length?
Answer: 20 cm each
This standardized coloring aids in visibility and estimation of intermediate distances during ranging operations.
17. What is the primary advantage of a steel tape over a chain for linear measurements?
Answer: Higher precision and flexibility for detailed measurements.
Steel tapes provide finer subdivisions and are less prone to distortion than chains, leading to more accurate readings.
18. If a survey line passes through a building, rendering direct chaining impossible but allowing ends to be visible, what auxiliary equipment/method might be used?
Answer: Ranging rods and principles of similar triangles or perpendicular offsets.
This is an obstacle to chaining only. Ranging rods help maintain alignment, and geometric principles are used to calculate the obstructed length.
19. For mapping small, open areas where only linear measurements are taken, the most suitable surveying method is:
Answer: Chain Surveying
Chain surveying is specifically designed for such conditions, relying solely on linear measurements for plotting.
20. What is the main purpose of galvanizing the links of a surveyor’s chain?
Answer: To prevent corrosion.
Galvanization provides a protective zinc coating, making the mild steel resistant to rust and extending the life of the chain in field conditions.
