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Rod (unit)

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A rod is a historical unit used to measure length and area. It is also known as a perch or a pole.

In the British imperial and US customary systems, one rod is exactly 16.5 feet. This is equal to 5.5 yards or one quarter of a surveyor's chain. In the metric system, a rod measures exactly 5.0292 meters. This unit is particularly useful for land surveying. This is because integer multiples of the rod can form an acre. An acre is a standard unit of area.

Gortavaura - geograph.org.uk - 1258792
Gortavaura - geograph.org.uk - 1258792

To understand how the rod works, we must look at its relationship to larger units. A surveyor's chain is a larger unit of length. One chain measures 22 yards or 100 links. It is exactly four rods long, which is about 20.1168 meters.

Münster, Historisches Rathaus, Preussische halbe Ruthe -- 2017 -- 9783 (crop)
Münster, Historisches Rathaus, Preussische halbe Ruthe -- 2017 -- 9783 (crop)

These units build upon one another to define distance. For example, ten chains make one furlong. A furlong is one-eighth of a mile. There are 80 chains or 320 rods in a single statute mile. This mile is 1,760 yards or 1,609.344 meters long.

These measurements create a precise system for calculating area. A perfect acre is a rectangular area of 43,560 square feet. It is bounded by sides that are 660 feet long and 66 feet wide. This is also described as 220 yards by 22 yards. In terms of rods, an acre is 40 rods by 4 rods. This means an acre contains exactly 160 square rods or 10 square chains.

Gortavaura - geograph.org.uk - 1258792
Gortavaura - geograph.org.uk - 1258792

Because of these exact ratios, surveyors can quickly calculate land sizes.

The history of the rod is very long. The name perch comes from the Ancient Roman unit called the pertica. In Rome, the pertica was 10 Roman feet, or 2.96 meters. In the 1500s, land surveying changed in England. King Henry VIII seized lands from the Roman Catholic Church in 1536. He needed to survey and sell this land quickly to fund wars.

A Meat Stall with the Holy Family Giving Alms - Placard
A Meat Stall with the Holy Family Giving Alms - Placard

A monk named Richard Benese wrote a book on surveying. He suggested using a rod with knots on a cord. Later, surveyors used rods made of wrought iron. These iron rods could be linked together to form a chain. This made the tools easier to carry through thick brush.

Different regions used different versions of the rod. In Ireland, the perch was standardized at 21 feet. This made the Irish chain and mile 27.27% longer than the English version. In the 13th century, perches varied greatly in length. Even in 1820, a House of Commons report noted many different sizes. In continental Europe, many local names existed for similar units. These included the canne, ruthe, and pertica. For instance, a Prussian Half Rod was 1.883 meters long.

Münster, Historisches Rathaus, Preussische halbe Ruthe -- 2017 -- 9783 (crop)
Münster, Historisches Rathaus, Preussische halbe Ruthe -- 2017 -- 9783 (crop)

Modern technology has changed how we measure the world. The rod has been replaced by steel tapes and electronic tools. Surveyors now use lasers and optical target devices. In the United Kingdom, the rod was phased out during metrication starting in 1965. In the United States, the definition of the rod changed over time. Until January 1, 2023, it was often based on the US survey foot. Today, the National Institute of Standards and Technology uses the international foot of 0.3048 meters.

Even though it is less common, the rod is still used in specific ways. In recreational canoeing, maps use rods to measure portage paths. A portage is a path where a canoe is carried overland. This is helpful because many canoes are about one rod long. In real estate, the term is used for pipeline easements. Offers for these easements are often expressed as a price per rod. In some places, like Jamaica and Sri Lanka, the perch is still used in property titles. It remains a small but important part of how we define the ground beneath us.

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File:A Meat Stall with the Holy Family Giving Alms - Placard.jpg
A Meat Stall with the Holy Family Giving...
File:Münster, Historisches Rathaus, Preussische halbe Ruthe -- 2017 -- 9783 (crop).jpg
Münster, Historisches Rathaus,...
File:Gortavaura - geograph.org.uk - 1258792.jpg
Gortavaura - geograph.org.uk - 1258792.jpg
File:First Settlement marker - Northfield, Massachusetts - DSC07572.JPG
First Settlement marker - Northfield,...
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