|Article by Alan Pack|
The Gunter’s Chain
A chain is formed of 100 pieces of straight wire, the ends of which are bent to connect to each other through small oval rings, which for flexibility should number three at each joint. The wire is of iron or steel of from 8 to 12 gauge (.16 to .10 inches thick), the use of steel enabling weight to be reduced without sacrificing strength. The ends of the chain consist of brass handles, each with a swivel joint to eliminate twist.
A Gunter’s chain (named after its inventor Edmund Gunter 1581-1626) is 66 feet long and is divided into 100 links, each 7.92 inches long. The length of a chain is the total length from outside to outside of the handles. At every tenth link is attached a distinctive tag or tally of brass of the patterns shown in the figure. As each tag represents its distance from both ends of the chain, either handle can be regarded as the zero, so that a little care is necessary to avoid misreading. In taking readings between tags, one must count the number of links from the previous tag, estimating fractions of a unit if necessary. The use of the chain for surveying was largely discontinued during the first half of the 20th century. However although the chain was ideal for measuring over rough arable and grassland, there was a problem that, after considerable use the links in the chain became extended making it necessary to check and correct its length against a standard regularly.
The chain was much used when calculating areas of land during the ‘enclosures’. I did use a chain regularly in the late 1940’s and found it to be a very useful measuring aid particularly when working in long grass or very muddy areas.
Chains of 100 feet in length were made where the links were one foot long.
An excellent description of the use of the chain is given in ‘A Complete Treatise on Practical Land-surveying’ published in 1837 (Designed chiefly for the use of schools and private students) from which the following extracts are taken.
DIRECTIONS and CAUTIONS to YOUNG SURVEYORS when in the FIELD, etc.
Chains, when new, are seldom a proper length; they ought always, therefore, to be examined; as should those, likewise, which are stretched by frequent use.
In addition to the instruments already described, you must provide ten arrows, each about a foot in length, made of strong wire, and pointed at the bottom. These should be bent in a circular form at the top, for the convenience of holding them, and a piece of red cloth should be attached to each, that they may be more conspicuous among long grass, etc.
Let your assistant or chain-leader take nine arrows in his left-hand, and one end of the chain with one arrow in his right; then, advancing towards the place directed, at the end of the chain, let him put down the arrow which he holds in his right-hand. This the follower must take up with his chain-hand, when he comes to it; the leader, at the same time, putting down another at the other end of the chain. In this manner he must proceed until he has put down his tenth arrow; then, advancing a chain farther, he must set his foot upon the end of the chain, and call out, “change”. The surveyor, or chain-follower, must then come up to him, if he have no offsets to take, and carefully count to him the arrows; and one being put down at the end of the chain, proceed as before, until the whole line be measured.
Each change ought to be entered in the field-book, or a mistake of 10 chains may happen, when the line is very long. The chain-follower ought to be careful that the leader always puts down his arrow perpendicularly, and in a right-line with the object of direction; otherwise the line will be made longer than it is in reality. The follower may direct the leader by the motion of his left-hand; moving it to the right or left, as circumstances require, and always placing his eye and chain-hand directly over the arrow which is stuck in the ground. The leader likewise, as soon as he has put down his arrow, ought to fix his eye upon the object of direction, and go directly towards it. This he may easily effect by finding a tree or bush beyond the station to which he is going, and in a straight line with it and himself.
In hilly ground, if the follower lose sight of the mark towards which he is going, he must stand over his arrow; and the leader must move to the right or left, till he sees the follower in a direct line between himself and the mark from which they last departed.
If the surveyor can conveniently procure two assistants, the one to lead the chain and the other to follow it, it will be much to his advantage; as he will thus be left at liberty to take offsets, note down dimensions, etc. without loss of time.
Areas on modern 1/2500 scale Ordnance Survey maps are given in Hectares to three decimal places and, beneath that, the equivalent areas are given in Acres to two decimal places.
Calculation of an Area Using a Gunter’s Chain and converting the area to Acres, Roods & Perches
Survey, and make a scale plan, of the area to be calculated.
Divide the area into a series of triangles and draw in the verticals for each triangle.
Measure on the plan the base line and vertical for each triangle (in links) using the same scale as that used to produce the plan.
Calculate the area in links and move the decimal point five places to the left.
Rarely, if ever, were fractions of a perch used. This amounted to 5.5 yards square and, by rounding up or down to the nearest whole number; the measurement would be within 2.75 yards square of the true measurement, and of little consequence