Chain Surveying In Civil Engineering: A Comprehensive Guide to Precise Measurements

Chain In Surveying:

• In surveying, a chain refers to a measuring instrument used to determine distances on the ground. The chain used in surveying is commonly known as a surveyor’s chain or Gunter’s chain. It consists of a series of interconnected links, typically made of metal, with a standardized length.
• The most widely used chain in surveying is Gunter’s chain, which consists of 100 links. Each link is approximately 20.1168 meters (66 feet) in length. The chain is made of durable material, such as steel, and is marked at regular intervals to facilitate accurate measurements.
• The chain is designed to be used in a linear fashion, with one end held at the starting point and the other end extended along the survey line. The surveyor moves along the line, keeping the chain taut and aligned with the measuring line. The distance between two points is determined by counting the number of full chain lengths (100 links) and any remaining links.
• To enhance accuracy, the chain is equipped with handles or rings at each end for surveyors to hold securely. It is crucial to ensure that the chain is properly stretched and aligned during measurements to minimize errors. Surveyors must also account for any sag or variation in the chain’s length due to environmental factors like temperature and tension.
• The chain is particularly useful in situations where precise measurements are required but high-tech equipment like total stations or GPS receivers may not be available or feasible. It provides a straightforward and reliable method for measuring distances in a variety of surveying applications, including land surveys, boundary determinations, and topographic mapping.
• In modern surveying practices, the chain has been largely replaced by electronic distance measuring devices (EDMs) and Global Navigation Satellite Systems (GNSS). However, the chain remains an essential tool in situations where traditional surveying methods are still employed or as a backup option in case of equipment failure.

Terminologies In Chain Surveying:

In chain surveying, there are several terminologies and terms that are commonly used. Here are some key terminologies in chain surveying:

1. Chain: The chain refers to the measuring instrument used in chain surveying. It consists of a series of interconnected links, typically made of metal, with a standardized length. The most common chain used in surveying is Gunter’s chain, which consists of 100 links and has a length of 20.1168 meters (66 feet).
2. Link: A link is a single unit of measurement in a chain. In Gunter’s chain, there are 100 links, and each link is equal to 0.201168 meters (0.66 feet). Links are marked at regular intervals along the chain to facilitate measurements.
3. Offset: An offset is a perpendicular measurement from the main survey line to a point of interest. It is used to determine the position of objects or features that are not directly on the survey line. The offset is measured using a cross-staff or other measuring instruments and is recorded to accurately plot the positions on the map.
4. Station: A station refers to a marked point along the survey line. It is a reference point from which measurements are taken. Stations are typically marked using ranging rods or other temporary markers and serve as the basis for subsequent measurements and calculations.
5. Chainage: Chainage, also known as chaining or chaining distance, refers to the distance measured along the survey line using the chain. It is the cumulative length of the chain measured from the starting point or the reference station.
6. Traverse: A traverse is a series of connected survey lines that form a closed loop or polygon. It is used to determine the boundary or shape of an area. The traverse includes a sequence of stations, and measurements are taken between successive stations to determine the lengths and directions of the survey lines.
7. Bearing: Bearing refers to the horizontal direction of a survey line with respect to a reference point or meridian. Bearings are usually measured in degrees, minutes, and seconds clockwise from the north or south direction.
8. Compass Rule: The compass rule is a method used to determine the interior angles of a traverse. It states that the algebraic sum of the interior angles of a closed traverse is equal to (2n — 4) right angles, where n is the number of sides of the traverse.
9. Reducing the Bearings: Reducing the bearings refers to converting the observed bearings, which are measured relative to the magnetic north, to the true bearings, which are measured relative to the true north. This correction is necessary due to magnetic declination.
10. Closing Error: Closing error, also known as misclosure, is the discrepancy between the measured distance or angles of a closed traverse and the calculated values based on the geometric relationship of the traverse. It indicates the accuracy of the survey and can be used to adjust the measurements.
11. Main Survey Stations: Main survey stations, also known as primary control points, are fixed points established at key locations within the survey area. These stations serve as reference points for the survey and are typically marked with permanent markers such as concrete monuments or brass disks. Main survey stations are used as starting points for measuring distances and angles.
12. Main Survey Lines: Main survey lines are the primary lines along which measurements are taken in a survey. They connect the main survey stations and form the framework of the survey. Main survey lines are typically established based on the project requirements and the topography of the area being surveyed.
13. Check Lines: Check lines are additional lines that are measured to verify the accuracy of the survey. They are independent of the main survey lines and are usually positioned in a way that allows for cross-checking and validation of the measurements. Check lines provide an additional layer of quality control and help identify any errors or discrepancies in the survey data.
14. Offsets: In chain surveying, offsets are measurements taken perpendicular to the main survey line. They are used to determine the position of features or objects that are located away from the main line. Offsets are typically measured using a cross-staff or other measuring instruments and are recorded to accurately plot the positions on the map.
15. Tie Lines: Tie lines, also known as tie measurements, are additional measurements taken to connect the main survey lines or to link features of interest. They help to tie different portions of the survey together and ensure continuity and accuracy in the survey data. Tie lines can be used to cross-check measurements and provide additional reference points for plotting.
16. Base Line: The baseline is a long, straight survey line established within the survey area. It serves as the reference line from which other measurements and calculations are made. The base line is typically measured with great precision using specialized equipment and techniques. It provides the foundation for trigonometric calculations and the determination of angles and distances in the survey.

Understanding these terminologies is crucial for effectively conducting and interpreting chain surveys. They form the vocabulary and framework for accurately measuring distances, angles, and positions in the field, ensuring reliable and precise survey data.