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SETTING OUT BASES.
field numbered 42 ; from here we could perceive that a line A B would intersect our survey from end to end, would be free from obstructions, and that here and there we should pick up some offsetting upon it; the instrument was therefore planted, and the line_boned out, one man going towards A, and another towards B, so as to bone both ways at once; we have elsewhere spoken of the marks called “whites," and of ranging-rods; also of the necessity for these being set perfectly upright or vertical, often by means of plummet and line ; also of where on a base the mark
; should be placed so as to prove most useful. All these matters will be found fully described under the head of “Chain Surveying,"and the use of the theodolite will be found in our description of that instrument, so that we have no further observations to make here with regard to setting out or boning the base A B; this done, and whilst the man was returning from the far point A, we “ traversed”* the roads already mentioned, took the bearings of the base, of some of the interior lines of the traverse, also of the secondary line ab, the direction of which was fixed by the position of the fences adjoining it. Whilst doing this an opportunity was afforded of noticing the fields through which the junction of the proposed branch would run, and the theodolite being out, the line cd was boned, and the angle Acd measured, the commencement of a new base C D was boned out, and its intersection, by direction only, with a particular point on the . traverse carefully ascertained as one means of checking the direction of CD, besides taking the angle B C D.
The next operation is to chain the base A B, which may, of course, be commenced from either end, as happens to be most convenient; commencing, however, at A, the dispatch with which this operation is performed will depend in a great measure on the number of marks fixed during the boning; and as many of these may be merely “whites” planted on a bank in a hedge, there is no reason why there should not be one nearly, if not quite, for every field; for it will be observed that with the theodolite, when once planted, levelled, and securely clamped, these marks can be set in as fast as the men can be induced to run. Where this can be done from any eminence, so as to get the telescope to plunge as it were into the fields over the hedges, much time is afterwards saved in not having to cut gaps to clear the line of sight. With regard to fixing the stations, it may be observed in railway surveying, or in other cases where the side lines are short, that they may generally be fixed permanently at
* See article, " Surveying by Traverse."
once ; if we can observe from the base line, supposing vision unobstructed, how these side lines are to run, when a picket may be driven in, or if in grass lands a little triangle may be cut out, or a “crow's foot” mark may be made ; but if the lines extend such a length that a tall hedge or two obstruct the view, then it is better to make a “false station” or temporary mark only, which may generally be done by notching some stake in the hedge, and of course making an entry accordingly in the fieldbook. In every case mark on the ground as well as in the fieldbook every change of arrows, that is, every ten chains; in intersecting straight fences, stations are not required, as side lines will not be wanted, it being merely necessary to note both ends of such fences by offsets, and to take the angle they make with the base line; this is best done with the box-sextant or prismatic compass, as formerly described ; these fences are marked S in the field-book, and are thus known to be straight when we come to plotting. As regards stations for road-crossings, it will be found that to drive a picket at the foot of one bank, entering it as a "false station,” and making some mark on the hedge on the opposite side, such as cutting out a few branches or tying a piece of twine round a stake, will be as safe as anything ; driving spikes into the surface of the road, or breaking up a small hole and filling it up with stones, is a highly dangerous practice. There is but one further point to remark on as to the chaining of such a base line, and that is as to whether such chaining shall be done in portions or from end to end before commencing any other portion of the work. Supposing that we commence chaining at A in the first part of the day, the whole line measuring nearly 3 miles and a half, it will bring us very nearly to the close of the day's work on reaching the point B; and supposing we find an hour or two yet to spare, and begin some of the cross-lines at B, we shall then have to return a long distance to get back to quarters at the other end, unless we can find them near B, which, however, may after all be inconvenient. To add to this inconvenience, supposing we begin the cross lines at B, we shall have to return there in the morning to resume operations, or leave this part of the work unfinished, and commence at the more convenient point A, thus preventing the field-book from being as clear as it otherwise might be. If, on the other hand, due care has been taken to make the marks boned out on the base safe froin disturbance, and commencing at A we chain on as far as the first cross-road, and leave a blank page for the continuation of the base; then turn to the left down the road as far as a, and then to the left again along the line ab, which has been fixed by theodolite; in the next place lay out and chain
SETTING OUT BASES.
the triangles gf A and A hi, and the lines h k and kl, the latter of which has been fixed by the theodolite when taking the angles of the traverse; to complete this we have only short cross lines to run, so that we may soon clear off the work between A and the first cross-road; we may now run the line cd, which is produced both ways or at both ends, to fix lines outside the triangle cd e; next the line ed produced forward for ulterior purposes; the object in producing both these lines is that being two sides of a large triangle on the base line, tied into the traverse by numerous other lines, these two lines will be very useful if carefully produced to fix the base C D which was carried two miles beyond D, and also the base EF, of which we shall speak presently; the sides of this triangle were, moreover, checked by the line mln; there will be now no misunderstanding about filling in and taking up the remainder of the work west of the line cd. With regard to the base C D, we have already pointed out that it was fixed by the angle being measured at C, and by being sighted on to a station on the traverse, so that if C D had been wrong in position this would have shown it; it was further tied by measuring the lines C B and op from the fixed point o on the line cd produced, chained through p and q on the bases A B and CD; it was only necessary to take a little trouble in boning the line opq, so as to make it a good tie line at the same time that offsets are taken on it all the way. It has already been noticed that the base C D was produced two miles beyond D, and if such a line be out of position, so will also be the whole of the work upon it; it will also be seen that it is quite useless to employ a theodolite to set out a base, unless pains are afterwards taken to fix its position accurately.
We now come to the base E F; from E, a full unobstructed view of the country was obtained, and the theodolite was not got out; this base being set out to F, where the direction of the survey turned northwards, another base, GH, was at once set out, only the commencement of which is shown in our plan ; it will be readily seen how these bases are connected to the east of the plan by means of two small triangles; this was not considered a sufficient tie for determining on paper the direction of one base line with another; the line GH was therefore sighted backwards to I, so as to connect it securely with some back lines. It may now be observed that we have been fixing the base G H by tie lines to E F, before tying this last line which was first set out; this was because we were in that direction, and therefore determined these points at once when observed, and to save time when walking over the ground again, a most important gain towards the despatch of work, and always to be kept in sight,
The base E F itself is determined in position by the triangle Ets, and by the line u t, produced to T, then to tie into HG produced backwards to this point; there are two further lines run jo v, wx, on which offsets are picked up; and now by means of cross lines, and others more or less parallel to the base, this portion of the work was completed; these parallels to the bases are often extremely useful in taking up fences, and are very conveniently set out by means of the box-sextant.
To the student we would suggest the tracing of this plan, leaving out the chain-work, which is represented by the dotted lines; that he should next with the plan and text before him lay down the base lines and their connecting ties in the manner above described, and keeping in view the same motives, he will then have a better opportunity of distinguishing how these lines are, as it were, framed together, laying at first loose and uncertain to some extent, until gradually they become all locked together by the tie-lines: when once satisfied that this is fully accomplished, he need not lay out any more, but fill in as quickly as may be; that is, set out the remaining lines to complete the offsetting, by chaining from station to station, in the easiest manner presented by the nature of the ground and the fences. In carrying out the above suggestion the student need not apply himself, to lay the chain-lines on the tracing-paper exactly or minutely as in the example; it will only be necessary for him to do this to such an extent as to satisfy himself as he proceeds that he understands the rationale of what he is about. Let him remember also that no two cases are exactly alike; that all, therefore, will require treating in a manner different more or less.
In running a line we sometimes unexpectedly find that, to carry it on, we must chain on a fence, or abandon it, as in Fig. 78; in such a case, it is very simple to lay out a parallel line for a length sufficient to overcome the difficulty, and return to the main line when this is done ; ab, c d, e f, g h, are all equal, and set out at right angles to the line A B; d f is therefore parallel and equal to ce; a sketch and memorandum of this is of course made in the field-book.
When buildings are numerous on such work, we advise, to avoid the false economy of being sparing with chain-lines at the expense of long offsets
, thereby going over the length of chain-line perhaps a dozen times; as an invariable rule, the nearer your work, the shorter your offsets, the better you can see to sketch the plans of your buildings, and the more correct the offsets will be in length and direction; it is also to be remembered that the angles and sides of buildings are much more defined on the ground than hedge and ditch.
We may now close these remarks, as under the head of “Chain Surveying,” much more has been said referring to this as land surreying generally; but it should be observed that, in some parts of the plan by which we have illustrated the above remarks, a much greater width has been taken in than is generally done ; this partly arises from there being a junction of two lines in the middle of the survey, and partly from the line of rail being undetermined in position when the survey was made; the average maximum of width is ten chains on each side of the centre line, where more is required special instructions should be given for
An engineer, when making a survey, will take every opportunity of making himself acquainted with as many geological details
, or subsoil information, as can possibly be acquired during the survey; he will also acquire of his own knowledge, as much information as possible as to the effect of floods on brooks and streams, the span of arches over them, and the diameters of culverts; the widths and inclination of roads; the kind and cost of building materials, and, if opportunity offers, any information as to traffic along the roads.
In making an exploring survey, the engineer's object is to ascertain the general direction of the route along which the best paying and most cheaply constructed line may be found; at least where neither the one nor the other of these important elements have to be sacrificed to some local interest, instances of which in this country have been but too flagrant; with a good general map, an exploring survey may almost be resolved for many purposes into a local examination, and noting certain lines on the ground from which a selection may afterwards be made according to circumstances, as they become developed, either as regards levels, construction of works, traffic, and local interests. But where there is no such map, it is not detail we have to explore for, but the general features of the country ; in this sense, a survey of the details only would be so much waste paper, and under any circumstances would be at first, waste of time. A few main lines showing the general position of rivers, streams, roads, or tracks, woods, forests, villages, valleys, and summits of hills, with their general angle of inclinations and altitudes, with notes as to geological features; these are the points on which the engineer requires information, and it will be quite sufficient if they are shown on a map of from one to three inches to the mile; until the direction of a line proposed is determined on, a survey of details is of no value.
As regards surveys of railways and canals, much analogy will exist between both, as regards the different levels to be overcome,