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sighting a line you would be much better without it, as it is sure to make your work of no avail.
It may very truly be said that these observations are very tedious, and that to many they are unnecessary; but it must be remembered, that we are not writing for the experienced, but the student; strange as it may appear, it is on the carrying out of these details that the correctness of a survey often depends. Most generally speaking, unless some good general map or an old survey be at hand to guide us, we shall have, in order to fix our main stations, to walk twice, or perhaps three times, round the district to be surveyed; this will a good deal depend on the use we make of our eyes during such perambulation. Let us suppose, for instance, that we are starting from A, Fig. 5; the first thing to be done is to look in the direction of B, C, and E, if we can form any idea of their whereabouts on the ground, and we shall generally be assisted in this by making inquiries of some local men accompanying us as to whether such and such objects as may happen to be in sight are within the district to be surveyed, such as a house, a church, a mill, a clump of trees, &c.; then supposing vision is unobstructed before us, see how lines would lay from A; perhaps, however, on account of high ground in every way before us, we can see hardly anything; in this case we must be content with marking any object that may happen to lay about this corner of our survey; then walk on towards E or B, as circumstances may advise. On reaching another marked angle of the survey as E or B, we must endeavour to descry any of the objects we have just noticed about A, though even two or three chains off from A ; and look out for what we may see or guess at round about C and B; walk on to C and B, making similar observations, and if we judge it advisable, back to A. Should A, however, lay completely in a hole, it will probably not be of much use, unless to set up a temporary pole there, and this we should not probably do unless our walk along the boundaries between A and E, and between A and B, led us to form some rough idea of the position of the main station thereabouts. Perhaps, however, by walking in the direction, as far as we can guess, of B A or E A produced, we may be able to get a better view, when we should at once do so.
After this first walk we may suppose we have formed some rough idea of the triangles to be laid out, though perhaps we may not know within half-a-dozen chains whereabouts either A, B, C, D, or E will lay. Still, if we have kept our eyes open, and endeavoured by observation and inquiry to form some idea of the position of the main angles of our district, we shall be much better off than when we first started. Now that we have
SETTING OUT BASE LINES
ascertained some of the main features of our district, we shall probably be able to fix, at least temporarily, something like the position of the main stations, when we go round a second time, and perhaps fix them permanently, if we can obtain good views from either E, C, or B. Should there lie high ground anywhere in the centre of the district, and which we perceive is visible from all four points, A, B, C, and E, or even two or three of them, then we should at once proceed to such high ground, after having formed some idea of the position of our main boundary angles. When there, with a couple of assistants and a man or two, we may, by moving about a little, set up rods on one or two lines, until we form some approximation as to where the bases crossing the survey will lay. This may assist us a good deal if we have previously taken care to mark objects at about A, B, C, and E, or even if we have only left a pole at any of these points with a flag on it. Generally, on an extensive survey, a pocket telescope is required for this work. The writer has a small double one of Elliott's, which has often mainly assisted him in laying down base lines. If, however, it should so happen that such difficulties or obstructions lay in the way, that we cannot lay out two such triangles as A B E, and ACB, we may perhaps get one of them, and then, from whichever base happens to cross the survey, we may throw up a couple of triangles over the remaining portion ; and if we cannot do this, we must adopt another system of triangulation altogether. Therefore it is desirable to keep our eyes open all the way as we first perambulate the district, for although we may not then be able to fix upon points for main stations, we ought to be able to decide pretty nearly what system of triangulation we shall have to adopt. Generally speaking, it is not wise to be in too great a hurry to commence chaining on such a survey, nor indeed in any case, because we are then liable to lay down lines on which no offsets are to be taken, when, by shifting the line a little at one end or the other, or perhaps at both ends, we may get a line with a good deal of offsetting upon it, thereby saving the necessity for filling in lines, to say nothing of the general rule in surveying to avoid a multiplicity of lines, because the greater the number of lines the greater the chances of error. At the same time the value of a base is not to be endangered for the sake of a few offsets, and sometimes a little shifting will make angles more acute or obtuse than desirable, more particularly when they are to be fixed by intersection of arcs only.
Should the surveyor happen to be provided with a compass, the prismatic compass, for instance, he will find it of considerable
, assistance in the sighting out his approximate bases ; supposing, for instance, that on first starting from A, he proposes a line, A E, bearing 15° east of north, and that he has marked some object in the distance having that bearing; but having walked in that direction for some distance, perhaps half a mile, he sees before him something which makes it desirable to alter this course ; keeping the back point A in mind, let him move right or left to get a new line to accommodate the new position of things before him ; say this new line bears 10° east of north ; let him now turn round and see how a line 10° west of south will suit the ground left behind us, for 10° west of south will be a prolongation backwards of 10° east of north ; if he sees reason to approve of this new line, let him fix on some forward object having that bearing, and if he sees that he is likely, from the nature of the ground, to lose sight soon of the greater portion of the back part of the line, let him fix on two objects. Having moved further north he may have to repeat the same operation, and so onwards. This will not fix the position of a base line, but will often be of considerable service in getting an approximation to it.
To guard, however, the reader from falling into the erroneous idea that having an outline of a survey of a particular form, he is at once to assume that one special and particular system of triangulation must be adopted, we have at Fig. 6 repeated a similar outline to that at Fig. 5, with this difference, that there is a long narrow ridge of high ground running east and west, about the centre of the district; this may at once render necessary a system of triangulation entirely different from the former one.
On the first inspection of the ground we see both difficulties and facilities in sighting two bases through this district, over the high ridge A B; moreover, we perceive that from this high ground we get a command of the whole of our district, and that along this ridge we can lay down a base from which we can throw up triangles north and south, and also that we can lay out our lines to such advantage as to get a good deal of offsetting upon them. All this is sufficient to determine us on assuming the line A B as the principal base. The points A and B are not to be fixed of necessity exactly where they are, and on settling their position it will be our business to determine them so that along A C aud B D we may be able to pick up offsets in the filling in; the position of C and D must be settled with the same view, keeping, however, in mind, the superior
, consideration of laying out C D, if possible, so as to produce it both ways to E and F, so as to make it a good and fixed line for tying others into, as H E and GF, thereby including the
SUGGESTIONS AS TO FIRST STUDIES,
south side of our survey by as few lines as possible, and making the lines on the east side check those on the west ; to settle finally, therefore, on the points E and F, may take a little time, but the firm tying up of this half of our survey makes it worth while. Whilst laying out this line we must not neglect to look out as to the best position of C and D, with regard to the whereabouts of A and B, so as to get on A C and B D the best intersection of fences, and as much offsetting as may be ; the points H and G, as regards this south side of our work, are to be fixed with due consideration to the same purposes, though as to G it is of greater importance to fix this point, if possible, so as to make F G I one straight line, as the direction of G I will then be a settled matter, and when we come to describe the triangle H IG with the beam compasses, and the line GI is drawn, it must make one straight line with GF, thereby holding a check on this triangle, independent of the intersection at K, and the tie line J I, which unites the summits of the triangles HGL and HIG; if all this comes upon paper as measured on the ground, our triangulation is safe, and we may proceed to fill in by laying down lines from one side of our triangles to another, or by throwing up interior triangles and tie lines.
We believe that we have now given all the information possible upon this description of surveying, and we shall only have to add presently a few words about "chain angles.” For the reader to be able fully to understand and apply what we have above said upon the triangulation and chain surveying of an extensive district, it will be necessary that he should have previously obtained a little practice as to chaining, offsetting, the field-book, and other elementary steps; and we have been writing on the supposition that he has followed advice already given as to obtaining a little practice with the chain, at first merely chaining a line, and taking offsets to adjacent objects two or three times over, and having accomplished this satisfactorily, then applying himself to the survey of 200 or 300 acres of land ; if this has been done, it is hoped that what has been said above will not be without advantage.
We would also advise the young surveyor who is practising merely for the sake of study and improving himself, immediately he can, with tolerable ease and comfort, manage 200 or 300 acres, no longer to be satisfied with such limited practice, but to set about much larger areas, even if it be only as to the outlines and principal features; this will bring him more speedily than anything else into such practical experience as will most readily make him soonest useful ; an extensive common, for instance, with the circumscribing and intersecting roads, with houses and gardens by the way-side, pools, ponds, streams, &c.; if, instead of doing this, he devotes himself entirely to the practice of a few fields here and there, or a short length of road or two, with a few houses, he will acquire the practice of chaining and offsetting, and throwing up a few little triangles, but he will not acquire the habit of that large and comprehensive triangulation required in almost all such surveys as enter into engineering field-work in practice; and when he comes to the reality of this in the actual experience of professional life, he will feel at a great loss; but if, to repeat ourselves once more, he has taught himself to chain a long line, to read his chain as he proceeds, to keep his field-book, to take and enter his offsets, and sketch adjacent objects as he goes along; if he has learnt to do this correctly, and then, as we have said before, applies himself to surveys first of two or three fields, and gradually extends his work until he can manage two or three hundred acres, he will be perfectly able to try his hand upon two or three large tracts of sand—say a couple of miles long, and half as broad ; if he has also by this time acquired the habit of keeping his field-book, as shown in a former place, in such manner that there is no difficulty in reading and understanding, and therefore in plotting from it, he will
be fit for something in engineering field-work, as far as land-surveying is concerned. But to imagine that, having acquired the habit of measuring two or three fields, and a road or two, he can make himself useful in actual work, is to be led into a delusion from which he may be very unpleasantly aroused
a delusion which can only be fostered by those who are amusing themselves at his expense, and one from which he will find a bitter awaking when he finds that he is not even fit to be a chainman in actual work; if, on the other hand, he has acquired the practice by himself of working on large areas in one place, he can do it in another. The only grounds on which we would recommend the student surveyor every now and then to undertake small surveys would be, having the intention of plotting to a very large scale, say 30 or 40 feet to one inch, and then there should be plenty of offsetting and buildings, &c., all the minutiæ of which should be shown, such as gateways, steps, pavements, drain gratings, &c., in short, everything actually on the ground, and all of which must be shown on the plan. Such surveys demand great care and patience, for here an error of even six inches, or less, may be a serious matter.
Surveying by chain angles is a method of fixing the position of the chain lines by triangles set off upon portions of them. Fig. 7 is an illustration of this, where it is required to set out the