Chain surveying is one of the simplest and oldest methods of land measurement used in civil engineering. It is based entirely on measuring linear distances using a chain or tape without relying on angular measurements. This method is best suited for small, level areas with minimal obstacles. Understanding chain surveying is essential for civil engineers, surveyors, and construction professionals because it forms the foundation of modern surveying techniques.
What is Chain Surveying?
Chain surveying is a method of surveying in which only linear measurements are taken on the field. No angular measurements are involved, and the work is carried out with the help of a chain or tape. It is suitable for areas with simple details and flat terrain. This method is cost-effective, easy to carry out, and requires minimal equipment. However, it is not suitable for rough or hilly areas.
Equipment Used in Chain Surveying
In chain surveying, a variety of tools are used to ensure accurate measurement and mapping. The main equipment includes:
Chain or Tape
The chain is the primary instrument used for measuring distances. Common types are: Gunter’s Chain (66 feet), Metric Chain (20 m or 30 m), and Engineer’s Chain (100 feet). Nowadays, steel tapes are also used for more precision.
Arrows
Made of steel wire, arrows are used to mark the end of each chain length during measurement.
Ranging Rods
Tall poles with colored bands used to mark positions and ensure straight alignment between points.
Pegs
Wooden pegs are driven into the ground to mark survey stations or important points.
Plumb Bob
Used to transfer a point on the ground vertically, especially when measuring from elevated positions.
Cross Staff
A simple instrument for setting out right angles in the field.
Offset Rods
Used for measuring short offsets or perpendicular distances from a chain line to nearby objects.
Procedure of Chain Surveying
The chain surveying process follows a systematic approach to ensure accuracy and efficiency.
1. Reconnaissance
A preliminary survey is conducted to inspect the site, identify survey stations, and note obstacles. A rough sketch called a key plan is prepared.
2. Station Fixing
Survey stations are fixed on the ground using pegs, ensuring clear visibility between them. These stations form the framework of the survey.
3. Laying Main Lines
Main survey lines are measured and laid out using the chain and ranging rods. The longest possible lines are chosen to minimize cumulative errors.
4. Taking Offsets
Offsets are measured from the main survey lines to record details like buildings, boundaries, and natural features. Offsets can be perpendicular or oblique.
5. Plotting
Measurements are transferred to paper using a suitable scale. The plotting should be neat and accurate to represent the surveyed area correctly.
6. Checking and Adjustments
To ensure accuracy, check lines are measured between fixed stations and compared with plotted distances. Errors are corrected if necessary.
Errors in Chain Surveying
Errors in chain surveying can occur due to various reasons, and understanding them helps in minimizing mistakes. Errors are generally classified into:
1. Instrumental Errors
These occur due to defects in surveying instruments, such as a chain being too long or too short due to wear and tear or incorrect manufacturing.
2. Personal Errors
Mistakes made by the surveyor, such as incorrect ranging, miscounting chain lengths, or poor alignment, fall under personal errors.
3. Natural Errors
Environmental factors like temperature changes, wind, or uneven ground can affect measurement accuracy.
4. Cumulative Errors
Small errors that occur repeatedly in the same direction and accumulate over long measurements.
5. Compensating Errors
Errors that occur in both positive and negative directions and tend to cancel each other out over time.
Advantages of Chain Surveying
Chain surveying is simple and inexpensive, requiring minimal equipment and technical skills. It is easy to learn and perform, making it suitable for small-scale projects. The method is effective in open areas with few obstructions.
Disadvantages of Chain Surveying
It is not suitable for large areas, uneven terrain, or places with numerous obstacles. The accuracy is lower compared to modern instruments like total stations. Measurements can be affected by weather conditions and human errors.
Applications of Chain Surveying
Chain surveying is commonly used for small land surveys, plotting boundaries, preparing site plans for small construction projects, and preliminary route surveys.
FAQs on Chain Surveying
What is the main purpose of chain surveying?
The main purpose is to measure linear distances accurately for mapping and construction purposes, especially in small, flat areas.
Can chain surveying be done on hilly terrain?
No, chain surveying is best suited for flat terrain as elevation changes cause measurement inaccuracies.
How to avoid errors in chain surveying?
By checking the chain length regularly, ensuring proper ranging, taking measurements in favorable weather, and following a systematic procedure.
Which chain is most commonly used?
The 20-meter metric chain is most common in modern surveying due to its simplicity and standardization.
