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Chain (unit)

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A chain is a specific unit of length used in the US customary and Imperial unit systems. It measures exactly 66 feet, which is also equal to 22 yards. In the metric system, one chain is equivalent to 20.1168 meters. This unit is subdivided into 100 smaller segments called links. A single link represents one hundredth of a chain, or 0.66 feet. This measurement is vital for surveying, which is the process of determining the exact boundaries and dimensions of land. The term chainage refers to the running distance measured along a survey line from a fixed starting point.

Bridge miles and chains
Bridge miles and chains

The mechanism of the chain allows for precise land division. Surveyors use the chain to create straight or curved lines across a landscape. By counting the number of links or chains, they can determine the exact length of a property boundary. In the United States, this was used to define parcels of land in the Public Land Survey System. For example, a section of land is approximately 80 chains on each side. A quarter-section is about 40 chains on a side, and a quarter-quarter-section is about 20 chains on a side. This system creates a predictable grid for ownership and mapping.

There are several different types of chains used throughout history and different regions. The most famous is Gunter's chain, which is 66 feet long with 100 links. In the late 18th century, Jesse Ramsden developed the engineer's chain, which was longer at 100 feet but still contained 100 links. In the Southwestern United States, surveyors used the Texas chain, which measured 20 chains in length. India utilizes metric chains in specific lengths, such as 5, 10, 20, or 30 meters. India also uses a specific revenue chain for cadastral surveys that is 20 meters long and contains 16 links. In North America, forestry workers sometimes use a modern version called a static cord, which is a 50-meter rope marked at specific intervals.

The history of the chain is tied to the development of accurate land mapping. The surveyor's chain appears in a Dutch map from 1607 and an English book from 1610. In 1620, the polymath Edmund Gunter created his 66-foot method to improve land surveying accuracy. This 66-foot unit was previously known as four perches or rods. By 1675, Gunter's system became the standard for surveyors and spread to the American colonies. In 1784, Thomas Jefferson proposed a rectangular survey system for the Continental Congress. This led to the Land Ordinance of 1785, which mandated the use of the chain for official government surveys in the United States.

The significance of the chain is seen in how it organized entire nations. In the United States, the chain was mandatory for laying out townships. In the United Kingdom, different versions once existed before the imperial system was adopted in 1824. Before this change, a Scots chain was about 74 feet and an Irish chain was 84 feet. In Australia and New Zealand, the chain influenced urban planning and property sizes. Many building lots were historically a quarter of an acre, measuring one chain by two and a half chains. Even today, some street frontages in these countries remain one chain wide.

Notable examples of the chain's survival can be found in modern infrastructure and sports. In the United Kingdom, the chain is still used on railways to identify locations. A bridge might be located at a specific mileage and chainage, such as 112 miles and 63 chains from a starting station. In North America, agricultural workers use measuring wheels with a circumference of 0.1 chain. Wildland firefighters also use the chain as a unit to measure the rate at which a wildfire spreads. Interestingly, the length of a cricket pitch, the distance between the stumps, is also one chain.

The concept of the chain connects to various fields like civil engineering, forestry, and law. In New Zealand, the "Queen's Chain" refers to a strip of public land, often one chain wide, set aside for public use along coasts and rivers. In forestry, specialized tools like the hip-chain, a small box with a string counter, help workers measure distance. The chain's legacy is also found in legal descriptions of land known as metes and bounds. Whether used in a subway design or a rural road name, the chain remains a fundamental part of how humans measure and divide the world.

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File:Bridge miles and chains.jpg
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