Section I.

PART III.

SETTING-OUT.

SETTING-OUT ON THE STRAIGHT.

SETTING-OUT, or the location of intended works or of any predetermined points, is that branch of geodetical operations which is the converse of surveying and levelling in their exact sense, which consists in discovering the position of a series of actually-existing points, and through them forming a correct representation of existing features. Setting-out points as regards level differs little from levelling, as it is so much dependent on existing points that the operations are nearly the same. in each case; the principal difference being that the exact points levelled must be marked by a peg or other means that will indicate correctly the position; this branch of the subject therefore requires but little further notice. Setting-out points, as regards position corresponding to design on plan demands that their relative positions shall be correctly marked on the ground. The greater portion of such ranging or setting out of engineering works is in straight lines, of long extent as centre-lines, and in transverse straight lines at right angles to these; most of the remainder is in straight lines at various angles and on circular curves having various radii.

Setting-out on the straight.-The setter-out, being in

all cases an adept at the use of surveying instruments, and competent to execute surveys and make trigonometrical calculations, requires but little additional experience, thought, and care to execute his duties in this branch; he simply applies the principles with which he is well acquainted in the most ready manner, with an amount of accuracy well suited to the requirements of the circumstances; sometimes the operations require little or no reflection, sometimes what is called usual practice-or the stereotyped habits of others—may be advantageously followed and sometimes advantageously improved upon. A few special tables for various purposes are occasionally useful; and with these, his instruments, and a few chainmen, the surveyor can undertake the duties of setting-out works.

Setting-out short distances on the straight can be effected by simply ranging three or four straight poles in the straight line, and taking the necessary measurements with a chain: right angles may be set out with the ordinary cross-staff, and one or two check measurements used to verify the resulting marks or pegs, which should be firmly driven in. The date of setting-out detached works, bridges, culverts, &c., should be invariably recorded at once, as there is a weakness among foremen and contractors' agents for removing such marks, and either declaring that none ever existed or eventually making fresh ones for themselves and declaring them to be the original marks: the former plan to justify delay, the latter to absolve themselves from blunders. Ranging in straight lines with poles may be done for several miles by very exact persons with hardly any error; but the attempt is hardly to be recommended generally. The permanent marking of extensive works,

such as those of centre-lines of canals, railways, or roads, is generally effected with a theodolite; although any good telescope with wires will range points in one direction, yet for ranging both ways a theodolite is required that will reverse well, and a transit theodolite is generally to be preferred, as the correctness of ranging would be entirely spoilt by a slight bend at the point of observation. When a Y theodolite is used for such purposes it is not sufficient to trust to the graduation at 180°, the telescope should be taken out of its bearings and reversed in them, and a sight taken on the same object by way of test. In ranging very long straight lines, such as occasionally occur on canals or railways in flat countries, which may be even fifty miles in length, a portable transit-instrument should be used in preference to a transit-theodolite; the ranging of very distant points should always be done with it, while intermediate or smaller distances can be ranged with the theodolite. The setting up of the transit-instrument, and its corrections and adjustments, are treated in the chapter on Astronomical Observations on Route Surveys.

Obstacles, such as pieces of marsh-land, immense trees, etc., sometimes occur in lines of long alignment, which impede the vision and obstruct measurement; the operations and devices used in passing such obstacles are very similar to those adopted in ordinary chain survey.

Points of given level.-Sometimes it is customary to have only one set of ranged pegs or marks both for position and level, the level points being marked on these; but in many cases it is necessary to have a second set of pegs to denote level. Level-marks are required at every change of gradient, at crossings, and

at all masonry works or special constructions; permanent bench-marks should be set at about every quarter of a mile over extensive works; a description of their positions should be recorded; and no error of more than an inch or two per mile allowed to remain permanently.

The permanent stakes used on centre lines of works of communication are generally placed at distances of 100 feet apart throughout the whole line from the commencement; in towns or crowded localities they are interpolated by pegs at every 50 feet apart, and on sharp curves at 25 feet apart. On some works, and in most English railway-practice, where Gunter's chains and furlongs are preferred, the distances of the stakes. are taken at similar convenient distances expressed in those units. Such stakes are of uniform pattern, square in section, 3 feet long, painted and capped and numbered. In open country their distances apart may be increased to 400 feet, or to 5 Gunter's chains. The permanent level-stakes, if required, should be round, of a slightly different length, and otherwise painted so as to be easily distinguished.

When the land is tolerably flat, the staking and setting-out on the straight is comparatively simple and rapid; but on embankments, in cuttings, tunnels, or places where there is water, difficulties present them selves. The true position of a stake in cutting can only be arrived at when the cutting is completed, and any temporary stake would be liable to removal as the work proceeds. Hence the usual mode is to set out two lateral marks clear of the cutting on each side of the centre line and at right angles to it; the point in a cord strained between the two marks can then be plumbed

down to obtain the true position of the permanent stake at any time; while as regards level, if any two level-marks set to the proper gradient are given at each end of the cutting, a foreman can use boning-staves on them to obtain the level at any required point, and thus regulate the depth of excavation.

For high embankments lateral points are set out in the same way, being marked by tall poles with crosspieces nailed to them, showing the intended final level; the permanent stake is set after the embankment is finished and has had a little time to settle.

In setting-out tunnels the centre line is generally first set-out on the ground surface; various points in it are afterwards plumbed down through the shafts to the required level, and the centre line set out afresh from the points below ground. As the shafts are rarely more than 10 feet in diameter this operation requires extreme nicety. The method adopted on the Metropolitan Railway was as follows. At each shaft the above-ground centre line was marked with great precision by two pegs each 25 feet distant from the centre of the shaft; in each peg a nail or eyelet was driven and sighted in with a transit theodolite; a cord strained between these nails then showed the exact position of the centre line over the shaft mouth. Two points were then marked on the cord, their distance apart being about six inches less than the width of the shaft, and these two points were plumbed down to the bottom of the shaft, where two corresponding permanent iron marks were set; thus giving the direction of the centre line below-ground. This operation was repeated several times to eliminate errors in the plumbing down, which at all times, and especially in windy weather, is tedious and doubtful in