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ENGINEERING FIELD WORK.
Land Surveying.–Description of Gunter's Chain.-Chain
ing and Practical Observations on the Method of Chaining correctly, and with Despatch. - Chaining on Sloping Ground.—Stepping and Inclinometer.-Off'sets.—Offset Staff.—Method of Offsetting.–Stations.-Field-book.–First Description of the details of a Survey.--Practical general
Remarks.-Straight-Edge. WHETHER it be proposed to construct such works as railways, or canals, docks, harbours, waterworks, roads, navigation or town improvements, large buildings, or any public or military works, a survey of the lands through or on which such works are to be constructed, is one of the first requirements, and this survey has to be laid down on paper according to certain adopted usage amongst engineers, and in conformity generally with some legal regulations of the country in which the survey is made, and which in England we term “Standing Orders ;" it is also often required for large or small districts of land, either for public or private purposes. A good survey is a faithful representation on paper of any particular lands, either as to the whole or part; it is correct in length and breadth, and shows accurately the position of every fixture on the land, natural or artificial, as rivers, streams, roads, fences, buildings, &c. &c.
There is more or less difference observed in making surveys according to the purpose for which the plan is required; for the construction of a railway, a canal, a road, or the improvements of a river, a long narrow slip is required; for a dock, for waterworks, for town improvements, and many other purposes, where as much in width as in length of land may be required, another method has to be followed, though the same principles are carried out; for a maritime survey, where a great portion of the work is what we term "inaccessible,” another system has to be adopted;
but they all resolve themselves into one thing, and that is, a correct plan of the lands and areas surveyed; although the methods pursued are different, the instruments are the same, and consist of certain lineal measures for ascertaining length and breadth, and certain instruments for measuring angles, whether the lines containing the angles are actually measured in length, or calculated, or merely laid down on paper according to the direction given by the lines containing the angles. For many military purposes, instead of using a chain, lines are sometimes paced by the military surveyor on foot, or sometimes on horseback; but for civil engineering, all linear measurements are taken with the chain, which is one of the most important of all our instruments, and with the use of which we must be perfeetly familiar in all its appliances before we can do anything in engineering field-work, as far as surveying and levelling are concerned, or setting out works. Before proceeding with a description of this instrument and its use, we must observe that we take it for granted that the beginner is acquainted with the ordinary use of a pair of compasses, a drawing pen, a T square, and a set square; if unaccustomed to drawing, the best thing he can do is to apply himself first to tracing, and then to copying some drawings; dividing a foot square of paper into square inches
, and these again into forty or fifty equal parts, horizontally and vertically, is very good practice; so is describing a hundred circles in one of two inches diameter. When he can readily do this, he will handle his compass, pen, and rule with ease and comfort.
The instrument generally used for taking land measurements is known as Gunter's Chain : it is 66 feet in length, and divided into 100 links, united each to each by three rings, usually of elliptic form. For very important and delicate purposes, where the sides of triangles are all calculated, a steel chain, jointed like a watch chain, may be used for measuring the base line, and which is always to be stretched to an equal degree; it is often laid in troughs, carefully adjusted horizontally, and allowances are made for extension and contraction from temperature ; field engineering, however, but very rarely indeed requires the use of this chain, and we shall at once return to the use of “Gunter's Chain.” This being 66 feet long, and divided into 100 links, each link will be 7.92 inches in length, or X12 = 7:92. Every
C8 x ten links are marked by pieces of brass, shaped something like the fingers of the hand, the first ten being noted by one finger, the second ten by two fingers, the third by three, and the fourth ten, or forty links, by four fingers; fifty, or the centre of the chain, is noted by a round mark. From the other end of the
chain, the same notation commences up to the middle of the chain again, so that one finger may note 10 or 90, accordingly as it is read from one end or the other; and in the same manner 20 may be 80, &c. &c. Beginners have therefore to be careful on this point, for very serious errors are often made, even by those who have had a good deal of practice.
An acre is equal to 4 roods, or 10 square chains, or 160 perches, or 4840 square yards, or 43,560 square feet, or 100,000 square links, and a length of 10 chains, by a width of 1 chain, is equal to an acre; or 1000 links, multiplied by 100, equal to 100,000, equal to one acre. If, therefore, the contents of a field, or of several fields, or of an entire survey, be found in square links, then dividing by 100,000 will give the acres ; but this division is effected by pointing off the five last figures towards the right hand, when the remaining figures will give the acres; multiplying the figures cast off by 4, and again pointing off the five last on the right, will give the roods; and repeating this operation with 40 instead of four, will give perches and decimals, thus :
Links. Links. A.
A. R. P.
28.00000 or 4-2-28.00000. At each end of the chain is a brass handle, by which it is held when in use. With the chain must be provided ten arrows, made of stout wire, about fifteen inches long, pointed at one end for the purpose of driving into the ground, and with a ring at the other end for hanging on the finger.
The chain is used for measuring distances straight from one point to another, and which may be only one or two chains in length, or five or six miles, or more; in measuring a line of five miles, or 400 chains (eighty chains being equal to one statute mile), if the chain be only one inch too long or too short, we shall have an error of 400 inches, or half a chain, and so in proportion as the line measures more or less ; hence the necessity for the length of the chain being correct. To make certain of this, a chain's length of 66 feet should always be set out on a level piece of ground, or along the coping of a wall
, so as to afford opportunity of testing the chain by this standard as often as necessary. Even with an old chain, which may be supposed to have been well stretched by repeated use, this testing should
always be performed once a week ; with a new chain oftener, and care should be taken to straighten the links that have got bent. When it is remembered that a chain may be stretched tight some three or four hundred times a day, or more ; dragged over rough ground, loaded with wet clods, through hedge and ditch, it will be easily perceived that a considerable elongation may take place; this should be corrected by shortening equally on each side of the 50-mark until the chain is reduced to its proper length, either by shortening two of the links, or taking off a couple of the rings.
The operation of using the chain is performed by two persons, called the “leader" and the “follower," and in the manner now to be described. The leader starts in the direction required with the ten arrows in his left hand, and one end of the chain in his right, the follower remaining at the starting point, there holding the other chain end. Having dragged out the length of the chain, the leader holds one of the pins or arrows with the handle of the chain in his right hand, and according to the direction of the follower, sticks it into the ground on the line to be measured. This done, and on a signal being given, the leader leaving the first arrow sticking in the ground, and giving the chain a cast to the right to prevent its dragging against this arrow, proceeds until he. bas reached another chain's length ; whilst the follower walks up to the pin first driven in, at which he holds his end of the chain whilst the second arrow is stuck in the ground by the leader according to the follower's directions. This operation is repeated until the leader has expended his ten arrows, upon driving the last of which into the ground, he calls out “ten. Ten chains have thus been measured, and the Surveyor makes a note of this in the field-book. The leader now waits until the follower walks up to him; the latter makes a mark in the ground where the tenth arrow has been fixed, both leader and follower count the arrows to see that the number is complete, and the leader again taking the whole of them in his left hand, proceeds as before ; thus a line of any length may be measured on
a survey, care being taken never to omit noting in the field-book every tenth chain as reached, for which reason it is imperatively necessary that the leader call out “ten” every time the ten arrows are expended ; a fine, if necessary, will soon teach him.
The accuracy and dispatch with which a survey is carried on depend on no one thing more than on the chaining; for which reason it will be necessary to enter into a few details on this subject.
In chaining from one point to another it is necessary that the
as it is
chainmen should have at least one distinct object before them. On a line of any considerable length, as a base or sub-base, it will be necessary, in the first instance, that such a line be ranged out; that is, that poles be stuck up at intervals along the line, so that the chainmen may guide their work by such poles, or ranging-rods, which will have to be placed generally on the heights, and in the hollows, through which the line is to pass, so that, in crossing over the undulating ground, there shall be always one, if not two, of the ranging-rods visible in the foreground. If this is not attended to it will often happen that, on a sudden, the chainmen will find themselves without any objects to guide them. Generally, however, a good follower will look out for this, and select some object in a hedge, so as to keep himself in line until the poles become again visible. The straight stein of a tree, a poplar, a chimney, or any other well-defined object, should always be selected as a fore object, when convenient, as this selection, judiciously made, often saves a great deal of time, by preventing the necessity of sending men forward with poles. This is very often the case on lines of forty or fifty chains, and even more. For short lines, some well-defined object in a bush, a gate-post, a stake in a hedge, may be selected; but it is absolutely necessary that the object be well defined, so as to prevent mistakes, which might prove very serious. Occasions often occur when it is useful to leave two or three marks behind, by which the leader may be able to guide himself. A stick cut out of a hedge, with a cleft made in it, so as to hold a bit of white paper, often answers the purpose very well; all these little matters save time, as they avoid the necessity of fetching poles, and then sending forward or backwards to stick them up. These paper marks are called whites, and are often useful in sighting a line through a wood.
In order to insure good chainage-certainly one of the most important things in a survey-many good surveyors prefer “ following” the chain themselves; undoubtedly a good plan in many respects, but not without corresponding inconveniences, having to chain at the same time that he has to hold his fieldbook, and his “off-set-stuti," and, at the same time, having to keep his eyes about him, as to where his offsets have to be taken to, leaving stations, and making his notes. A little pains taken with chainmen, and a judicious enforcement of a little discipline, will almost always, in a few days, produce good chainmen. I will now endeavour to point out all that has to be observed to ensure good chainage.
On starting in the required direction, the leader should, instead of going at random, keep his eyes fixed on the forward