The road-network from William Roy’s Military Survey of Scotland. 1747
It is not known which type of measuring chain was used for the military survey, but we know it was not the Ramsden Chain (Engineers' Chain: 100 ft) since that was commissioned by Roy himself, later in his life. It seems very likely therefore that the instrument used was Gunter's Chain. This device was invented in 1620 and was in universal use from the 17th to the 19th centuries as the main measuring tool for land survey. It was employed extensively through the Americas, India and Australia as the empire expanded.
Gunter was a 17th century clergyman and mathematician. His invention was ahead of its time. In his day a long rod (or Pole) had been used to measure out land and compute area. This was of Roman origin. It had had been standardised, and we now know it was 16.5 feet long. Because Gunter found it cumbersome and unwieldy, he constructed a linked chain which was 4 Poles in length and thereby much more useful: a chain could be folded and carried. The key element was that the chain had 100 links; he further determined that 10 chain lengths would henceforth equal one furlong. Lengths could be summed in furlongs and expressed to three decimal points. This offered simplicity for anyone measuring long distances. Furthermore a strip one furlong long and one chain wide would be set to be one acre. Thus an acre could be divided into ten square chains. This, then, was the first decimal system, 150 years ahead of its time.
A Chain happens to be 22 yards long, and a furlong was one eighth part of a mile, but these conversions were of little relevance. All distances and areas could be expressed in furlongs , chains and links. It is interesting that a cricket pitch is 22 yards long and many road widths and block-lengths in erstwhile colonies are also 22 yards wide. Each link in a chain was about 8", this being the smallest unit of land measured length. On a Gunter's Chain there is a small metal label called a Tally, which denotes each 10 links: hence the phrase "tally-up". The chain was operated by two 'chain men' who stretched it as tightly as possible and inserted serial pegs on the outer border of the handle. They attempted to keep it as level as possible but there were also well used calculations to compensate for inclines and offsets.
Last updated Nov.2020