Understanding the Length of a Surveying Chain

The question asks about the definition of the length of a chain, specifically in the context of surveying. Surveying chains are traditional tools used for measuring distances.

Defining the Length of a Chain

A surveying chain is made up of a number of links, typically connected by rings. The measurement of the chain's length is standardized to ensure accuracy in surveying work. Let's look at the options provided:

• centre to centre distance between the last end of links: This option refers to measuring the distance between the midpoints of the handles or the last links at each end of the chain. This method accounts for the entire length of the chain as a measuring instrument.
• Sum of reduced level of benchmark and foresight: Reduced level, benchmark, and foresight are terms used in leveling, which is the process of determining elevations. This option is related to vertical measurements, not horizontal distance measurement using a chain.
• Sum of reduced level of benchmark and backsight: Similar to the previous option, this involves reduced levels, benchmarks, and backsights, all concepts from leveling. This is also unrelated to the physical length of a measuring chain.
• Sum of reduced level of benchmark and intermediate sight: Again, this option uses terms from leveling (reduced level, benchmark, intermediate sight). It is not relevant to defining the length of a chain used for distance measurement.

Based on the standard definition in surveying, the length of a chain is defined by the distance between the centers of the end handles (or the last links) of the chain. This ensures consistency regardless of how the chain is laid out, as the measurement starts and ends from defined points.

Detailed Explanation of Chain Length Measurement

Surveying chains are manufactured to precise lengths. For example, a Gunter's chain is 66 feet long, and an Engineer's chain is 100 feet long. The total length includes the handles at both ends. The standard way to define this length is the distance from the center of one end handle to the center of the other end handle. This distance is effectively the "centre to centre distance between the last end of links," considering the handles as extensions or the final links.

Therefore, the correct definition corresponds to the physical measurement of the chain as a tool for determining horizontal distances on the ground.

Incorrect Options Explained

Options 2, 3, and 4 all relate to leveling operations. Leveling is used to find the difference in elevation between points. Key terms used in leveling include:

• Benchmark (BM): A point of known elevation.
• Reduced Level (RL): The elevation of a point above a datum.
• Backsight (BS): A staff reading taken on a point of known elevation (like a benchmark or turning point) to determine the Height of Instrument (HI).
• Foresight (FS): A staff reading taken on a point whose elevation is to be determined.
• Intermediate Sight (IS): A staff reading taken on any point whose elevation is required, other than a backsight or foresight point.

Combining these terms with a benchmark's reduced level (e.g., RL of BM + BS) gives the Height of Instrument, not the length of a chain. Combining them in other ways (like RL of BM + FS) does not yield a meaningful surveying value related to horizontal distance measurement.

Thus, the options involving reduced levels, benchmarks, backsights, and foresights are irrelevant to the definition of a chain's length.

Revision Table: Surveying Chain Length

Additional Information on Surveying Chains

While less common now compared to measuring tapes and electronic distance measurement (EDM) devices, surveying chains were historically significant tools. There were different types of chains, including:

• Gunter's Chain (or Surveyor's Chain): This chain is 66 feet long and divided into 100 links, each 0.66 feet (7.92 inches) long. It is convenient because 10 square chains equal 1 acre, and 1 chain × 10 chains = 1 acre.
• Engineer's Chain: This chain is typically 100 feet long and divided into 100 links, each 1 foot long. It is simpler for calculations in feet.
• Metric Chain: These come in standard lengths like 20 meters and 30 meters, with links typically 0.2 meters long.

Each link in a chain is usually made of galvanized mild steel wire, bent into a ring at the ends and connected to adjacent links by three small circular or oval rings. This construction allows the chain to be folded and unfolded easily. The handles are usually made of brass or steel.

The accuracy of chaining depends on correctly measuring the length. Wear and tear or bending of links can alter the actual length, requiring calibration or adjustment. The definition focusing on the centre-to-centre distance of the end links provides a consistent reference point for measurement.