What Is Chain Surveying?
Chain surveying is a type of surveying in which only linear measurements are taken in the field. It is not possible to take angular measurements in chain surveying. Chain surveying is also known as chain triangulation.
Chain surveying is generally done for a relatively smaller area to describe the boundaries of plots or land and locate its existing features.
In chain surveying, the area that is to be measured is divided into a network of small triangles. Then the sides of each triangle are measured directly using a chain or tape. Only the linear measurements (lengths and widths) are measured, and no angular measurements are taken.
The triangles formed must be well-conditioned. A well-conditioned triangle is a triangle whose all angles are greater than 30° but less than 120°.
Principle Of Chain Surveying
i) To divide the whole area into a number of small and well-conditioned triangles.
ii) To take linear measurements of the sides of triangles.
The triangles formed on the field should be well-conditioned. As stated earlier, a well-conditioned triangle should have angles between 30° to 120°. If the triangles are equilateral, it becomes much easier for the surveyor to take linear measurements.
Instruments Used For Chaining
The following instruments are commonly used in chain surveying.
- Ranging rods
- Ranging poles
- Offset rods
- Plumb bob
Types Of Chains
Generally, five types of chains are used in chain surveying. They are as follows:
- Metric chain
- Gunter’s chain or surveyor’s chain
- Engineer’s chain
- Revenue chain
- Steel band
1. Metric Chain
Metric chains are available in lengths of 5m, 10m, 20m, and 30m. The links of the chain are made from galvanized mild steel wire of 4 mm diameter.
The two ends of the chain are fitted with brass handles with swivel joints. Swivel joints are provided so that the chains do not get twisted and jumbled up.
In 5m and 10m chains, the brass tallies are given at every 1m, 20m and 30m chains, the brass rings are provided at every 1m and brass tallies are given at every 5m.
2. Gunter’s Chain
Gunter’s chain is also called the surveyor’s chain. The length of Gunter’s chain is 66 feet. It consists of 100 links, each of 0.66 feet. A surveyor’s chain is suitable where the distances are measured in miles and the areas in acres.
3. Engineer’s Chain
Its length is 100 feet, and it consists of 100 links of 1 foot each. An engineer’s chain is used where length is measured in feet and area in square yards.
4. Revenue Chain
The revenue chain is 33 feet long. It consists of 16 links. It is used mostly for cadastral surveys.
5. Steel Band
A steel band is a ribbon made out of steel. On both ends of the steel band, brass handles are attached by swivel joints. The length of the steel band is 20m and 30m. It is 12mm to 16mm wide and 0.3mm to 0.6mm thick.
Step-wise Procedure Of Chaining
For chain surveying, two chainmen are required. The more experienced one stays at the zero ends or the rear end of the chain and is known as the follower. The other one who holds the forward handle is known as the leader.
Chaining involves the following operations:
- Fixing stations
- Unfolding the chain
- Measuring of distance
- Folding the chain
1. Fixing Stations
Stations are marked with the ranging rods so that they are also clearly visible from long distances.
2. Unfolding The Chain
To unfold the chain, the proper way is to keep both handles in hand and then throw the rest of the bundle of chains in the forward direction. Then the chain is laid straight.
The intermediate points are located by ranging if a survey line is longer than one chain length.
4. Measurement Of Distance
The follower holds the rear end of the chain at the starting point while the leader moves in the forward direction with the other end of the chain in one hand and a set of 10 arrows and a ranging rod in another hand.
When the leader is approximately one chain length away, the follower instructs him to fix the ranging rod in line with the pole.
When the point is ranged, the leader makes a mark on the ground and holds the handle with both his hands and pulls the chain so that the chain becomes straight between the terminal point and fixed point.
Then the leader puts an arrow at the end of the chain and proceeds further. A similar process is followed until the length of the ground is measured. The measured distance is recorded in the field book.
5. Folding The Chain
The chain is folded in a proper way after the measurement is done. The chain is folded by holding the pair of links at a time.
Errors In Chaining
The errors are in chain surveying classified as follows:
- Compensation errors
- Cumulative errors
- Personal mistakes
1. Compensating Errors
The errors occurring in either direction and tend to compensate are known as compensating errors. Compensating errors do not affect the results much.
2. Cumulative Errors
These errors occur in the same direction. They tend to accumulate. So, they are called cumulative errors.
3. Personal Mistakes
The errors occurring due to the carelessness of either follower or leader are known as personal errors. The leader or the follower is responsible for such errors.
Errors in chaining may arise from the following:
- Incorrect length of tape or chain.
- Improper ranging
- Carelessness in holding & marking
- Sag in chain
- Temperature variation
- Personal mistakes like misreading, miscounting by loss of arrows, reading of chain wrongly, etc.
Advantages Of Chain Surveying
- It is a simple method of surveying.
- Chain and tape are easy to use, and anyone can use them once they are given some basic training.
- The equipment used in chain surveying is cheap.
Disadvantages Of Chain Surveying
- It is useful only where there are no obstacles, such as plain ground.
- This method is time-consuming.
- If there is a variation in temperature, it may cause an error in results.