What is Linear Measurement in Surveying | Methods And Types Of Chains In Surveying | Chain Corrections in Surveying

Linear measurement in surveying is an essential aspect of surveying and civil engineering, used to measure distances between two or more points. Linear measurements are required in many projects, including construction, road and railway development, and land surveying. In this blog post, we will provide an introduction to linear measurement and discuss the different methods of linear measurement.

Introduction to Linear Measurement:

Linear measurement involves measuring the distance between two or more points, such as the length of a wall, width of a field, or distance between two buildings. Accurate measurements are essential in surveying and engineering projects because they provide the necessary data to create designs and plans for construction projects. There are different methods of linear measurement used in surveying and engineering, depending on the nature of the project and the equipment available.

Methods of Linear Measurement:

1. Chain Surveying: Chain surveying is one of the oldest methods of linear measurement used in surveying. In this method, a chain with a specific length is used to measure distances between two points. Chains are typically made of metal and come in different lengths, such as 20 meters or 30 meters. The chain is laid on the ground between two points, and the distance is measured by counting the number of chain links between the two points.
2. Tape Surveying: Tape surveying is another common method of linear measurement used in surveying and engineering projects. In this method, a measuring tape is used to measure the distance between two points. The tape is typically made of metal or fiberglass and comes in different lengths, such as 50 feet or 100 feet. The tape is stretched between two points, and the distance is measured by reading the markings on the tape.
3. Electronic Distance Measurement: Electronic distance measurement (EDM) is a modern method of linear measurement that uses laser or infrared technology to measure distances between two points. In this method, a handheld device is used to send a beam of light to a reflector at the other end. The time taken for the beam to travel to the reflector and back to the device is used to calculate the distance between the two points.
4. GPS Surveying: Global Positioning System (GPS) surveying is another modern method of linear measurement that uses satellite technology to measure distances between two points. GPS devices are used to determine the coordinates of two points, and the distance between them is calculated using mathematical formulas.

Approximate Methods Of Linear Measurement:

1. 1. Pacing: Pacing is one of the simplest methods of linear measurement and involves measuring distances by counting the number of steps taken. In this method, the surveyor takes a certain number of steps of a known length (usually their own step) and counts the number of steps required to reach the end point. The distance is then calculated by multiplying the number of steps by the length of the step. This method is typically used for short distances or in rough terrain.
   2. Rough Estimation: Rough estimation involves making an approximate calculation of the distance between two points by using visual cues and landmarks. This method is typically used for very rough estimates and is not suitable for precise measurements.
   3. Measuring Wheel: A measuring wheel is a device that consists of a wheel attached to a handle or frame. The wheel is rolled along the ground between two points, and the distance is measured by counting the number of wheel rotations. Measuring wheels come in different sizes and can be used to measure distances ranging from a few meters to several kilometers. They are typically used for measuring longer distances on smooth terrain.
   4. Odometer: An odometer is a device that is used to measure the distance traveled by a vehicle. Odometers are typically found in cars, trucks, and other vehicles, and measure the distance traveled by the rotation of the vehicle’s wheels. They can be used to measure both short and long distances.
   5. Speedometer: A speedometer is a device that measures the speed of a vehicle. Speedometers are typically found in cars, trucks, and other vehicles, and measure the speed of the vehicle by the rotation of the vehicle’s wheels. They can be used to measure the speed of the vehicle over a short distance.
   6. Pedometer/Passometer: A pedometer or passometer is a device that is used to measure the number of steps taken by a person. Pedometers typically clip onto a person’s clothing and measure the number of steps taken by detecting the motion of the person’s body. They can be used to measure the distance traveled by multiplying the number of steps by the length of the person’s step.

Special Types Of Chains In Surveying:

In surveying, chains are used for measuring distances between points. A chain is a long, flexible tape or rope that is marked at regular intervals and used to measure distance. There are several types of chains used in surveying, each with its own unique characteristics and uses. Here are some of the special types of chains used in surveying:

1. Gunter’s Chain: Gunter’s chain is one of the most commonly used types of chains in surveying. It consists of 100 links, with each link measuring 7.92 inches (20.1168 cm) in length. Therefore, the total length of Gunter’s chain is 66 feet (20.1168 meters).
2. Engineer’s Chain: Engineer’s chain is a longer version of Gunter’s chain, with each link measuring 1 foot (30.48 cm) in length. Engineer’s chain consists of 100 links, making it 100 feet (30.48 meters) in total length. Engineer’s chain is typically used for measuring longer distances, such as roadways, railways, and pipelines.
3. Steel Band: Steel bands are used for measuring long distances with high accuracy. They are made of steel and typically range in length from 50 feet (15.24 meters) to several hundred feet (over 100 meters). Steel bands are used for precise measurements, such as in geodetic surveys or large-scale engineering projects.
4. Revenue Chain: Revenue chain, also known as Gunter’s Surveyor’s Chain, is a variation of Gunter’s chain used for measuring land. It consists of 4 poles or rods, each measuring 16.5 feet (5.03 meters) in length, connected together by chains. The total length of the revenue chain is 66 feet (20.12 meters), the same as Gunter’s chain.
5. Engineer’s Chain (British): Engineer’s chain, also known as Ramsden’s chain, is used in the UK and consists of 100 links, each measuring 1 foot 7.92 inches (50.29 cm) in length. Therefore, the total length of British Engineer’s chain is 165 feet (50.29 meters).
6. Surveyor’s Chain (British): Surveyor’s chain, also known as Gunter’s Surveyor’s chain, is used in the UK and consists of 22 yards (20.12 meters) or 100 links, each measuring 7.92 inches (20.12 cm) in length. The total length of the British Surveyor’s chain is 22 yards (20.12 meters), the same as the Revenue chain.

Chain Corrections in Surveying:

In surveying, chain correction is a method used to compensate for the systematic error in measuring distances with a chain caused by the sag in the chain due to its own weight.

The chain correction is usually divided into two parts:

1. Correction For Standardization
2. correction for slope

1. Correction For Standardization

the chain correction for standardization is a method used to compensate for the error that may arise due to variations in the length of the chain. The correction is done by comparing the actual length of the chain with the standard length of the chain, and adjusting the measured distances accordingly.

The chain correction for standardization is calculated using the following formula:

Chain correction for standardization (Cc) = (Ls – Lt) x (L / Lt)

where Cc is the chain correction for standardization, Ls is the standard length of the chain, Lt is the measured length of the chain, and L is the length of the line being measured.

To use this formula, you first need to determine the actual length of the chain by comparing it to the standard length of the chain. This is done by measuring a known distance with the chain and comparing it to the actual length of that distance. Once you have the actual length of the chain, you can measure the distance of the line with the chain and use the formula to calculate the chain correction for standardization.

The resulting correction value can then be added or subtracted from the measured distance to obtain the true distance. It’s important to note that the chain correction for standardization is typically small, and its effect on the measured distance is usually negligible. However, for precise measurements, it is important to take all sources of error into account and apply appropriate corrections.

In addition to the chain correction for standardization, other types of corrections, such as correction for sag and correction for temperature, may also need to be applied to obtain accurate survey measurements.

2. Correction For Slope

Correction for slope compensates for the effect of sloping terrain on the measured distance. When measuring a distance over sloping ground, the horizontal distance is less than the slope distance, which is the actual length of the chain as measured. To correct for this effect, the chain correction formula for slope is used:

Chain correction for slope (Cs) = (L²/2H)

where Cs is the chain correction for slope, L is the length of the sloping line, and H is the vertical distance between the two ends of the line.

To use this formula, you need to measure the slope distance and the vertical distance between the two ends of the line. Once you have these values, you can plug them into the formula to calculate the chain correction for slope. The resulting correction value can then be added or subtracted from the measured distance to obtain the true horizontal distance.

It’s important to note that the chain correction for slope assumes a small slope angle, typically less than 10 degrees. For larger slope angles, a more complex formula may be required to calculate the chain correction. Additionally, the chain correction formula for slope is only applicable for surveying with a chain, and may not be necessary for more modern surveying techniques that use electronic distance measuring devices.