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PRECAUTIONS TO BE OBSERVED,
buildings are not right-angled or square built. This is often the case where it is not sufficiently so to be detected at once by the eye; and although the surveyor should never encumber himself or his men with things not absolutely necessary for the work to be done, it will often be found a very good plan to employ a boy to carry a large carpenter's square, by means of which it may be directly seen whether the angles of a building are square or not, when it may be treated accordingly.
A court-yard, through which there is no thoroughfare, will often take up a good deal of time before a sufficient number of lines are measured to ensure the production of a correct plan, as in Fig. 80 ; set out a line through the passage, making an angle AC D, with the chain line A B, and measure such angles with the theodolite, for they are too short to be measured with the sextant, on account of the principles of construction of the instrument; then measure the lines C a D, D b, ba f, fe, ec, cb, с а, a e, Df; a sketch should be made of this in the field-book, and the lengths of the lines written in upon them as they are measured. It is not to be supposed that all the work in a townland's survey will prove as troublesome as the two instances we have given in the illustrations, but it will generally be found to occur on inferior property, and the older portions of a town.
For a survey such as we are describing, there will be required a five or six-inch theodolite, a one hundred feet chain, and a tape of equal length, a couple of five-feet rods, half a dozen of six-feet poles, plummet and line, the square we have above mentioned, and a mason's chisel and hammer to cut station marks, and perhaps a few spikes to drive in where there is no horse traffic. With these will be required an assistant and three or four men.
With regard to the traverse, we recommend its being set out so as to include the whole of the survey, if not more than half a mile in length ; but whatever may be the extent of it, include as much area at a time as possible.
Supposing that you are going to traverse the whole survey at once, commence at some point where you can take as many angles or bearings as possible ; and if at a point to which you can see from other stations, or some other side of the traverse, so much the better. Make the sides of the traverse as long as possible, so as to have the least number of stations, and therefore the least number of angles to take; but do not fail at the same time to take angles or bearings down every street crossed by the sides of the traverse. Where there are many of these, there are two precautions which we recommend being taken,—the one is, that the station where the theodolite is to be planted for taking
these angles with the sides of the traverse after the whole of the bearing of the traverse itself shall have been taken, be sighted by the theodolite, and that such station be marked at once by a small “crow's foot” being cut with a chisel on the kerb-stone if possible, or some other mark made; the other is, that, to prevent such station falling upon a fraction of a foot, the length be measured out at once. Some trouble should be taken with some of these stations, and those on the other side of the work, so that it may be practicable to take observations from each to each.*
If the survey is too extensive to include the whole in one traverse, then commence at such a point that one of the sides of which you first take the bearing shall form a junction line with the next traverse, and if possible extend through the whole width of it.
Where any side of the traverse is of considerable length, we advise its being boned out with the theodolite at about every two or three hundred feet, by having marks cut on the pavement, or driving in spikes, if off the actual horse road. "The marks cut need not be more than a couple of inches long, if a memorandum be made as to the house or door opposite to which they are made.t Although some of these precautions will obviously cause a little more time, about a couple of hours generally, in taking the angles and setting out the traverse, it will
be found that afterwards time is saved in not having to bring out the theodolite again until wanted for the next enclosure, and also in chaining, as the "leader's” end is much more readily and accurately placed on the chain line when the fore object is only two or three chains in advance, instead of eight, or ten, or more. Indeed, our own practice for such work is to take or set out the length of the sides and the station lengths at first, at the same time the angles are taken ; thus avoiding entirely all fractions of feet. By means of these precautions at first starting, there is a very fair chance of being on the way to produce a survey that shall not be out so much as three inches anywhere.
With regard to objects to sight upon, the poles we have mentioned above are by no means constantly required for this purpose, and it is more desirable to chain on some permanent object, such as the angle of a building, a door-post, or any object near the ground; where none such occurs, a vertical line may be drawn in chalk on a wall, which will last a surprisingly long
* See what we have said on this head under the subject of " Surveying by Traverse.”
+ These marks must be crosses instead of crows feet, to distinguish them from station marks.
PRECAUTIONS TO BE OBSERVED.
while, but some slight mark should be cut besides for reference to hereafter.
In chaining, instead of the leader making a mark with a pin on the pavement, or paving, which is sure to be scarcely any mark at all, we recommend his being provided with a piece of chalk, with which he is to make a small cross at each chain's length. There are many cases where a hundred-feet chain may have to lay half an hour on the ground before being laid out again ; during this time passengers moving backwards and forwards, kick it out of its proper line; now by means of the two chalk marks, one at each, it is brought back immediately and accurately to the proper situation, both as regards length and direction. It is true the chainman should keep it stretched between the marks, but after laying any time on the ground it is difficult to make them do this, and the chalk marks we have just mentioned are much more simple and sure than trusting to them, for the surveyor can then easily see, from where he stands, whether the chain handles are in their proper place.
When the field-work has been completed, and the work plotted, which should be done more or less every day whilst all the objects are fresh in the surveyor's memory, it will be necessary to submit the work to a severe check, and this can almost always be done in the following manner :-Select two or three spots on the ground from which a view may be commanded of the whole, more or less ; such, for instance, as the roofs or flat leads of some of the loftiest houses ; find the exact points on the plan, and from these draw any convenient number of lines intersecting as many prominent features as possible, and examine with the plan on the spot how the work agrees with these points on the ground. It is generally quite possible to select some spot to which the theodolite may be raised, and on which it may be thus planted, when the plan may be submitted to any amount of scrutiny the surveyor may please, or judge necessary.
It may be necessary to observe that the above remarks on the working plan of a townland's survey refer principally to one plotted to a large scale, such as 30 or 40 feet to 1 inch, and on which therefore the general plan of the works of construction, such as a dock or station, a viaduct or cutting with its retaining walls, may be laid down with a considerable degree of precision ; if the plan of the survey is correct, it will at once be seen exactly what properties are and what are not required for the “emplacement” of the works ; and if the plan does not enable us to do this with accuracy, it is of no use, and if we are misled by it, then it is worse than useless. As an instance, a
long viaduct or cutting with retaining walls may be taken, such works may be commenced at once in a dozen different points, as required by the conditions of the structures, or accordingly as the company obtains possession of property required in as many different places. Unless we delay until the whole property is purchased, it is not possible to set out such a work, whether on a straight line or curves from end to end at once, and as such delay is not a practicable thing, it follows that the properties must be purchased, the line set out, and the works constructed on almost the sole condition of the good faith and working accuracy of the plan ; if at fault, it will follow that errors have been made in the purchase of property, and that awkward deviations have to be made in the works, and sometimes even that reconstruction may be a matter of necessity. There can, therefore, be no doubt that a correct working plan to a large scale is indispensable, though costly; that the expense of it will in the long run be an economy, and that such a plan can only be produced by the greatest care being given to it.
In a townland's survey for a general plan the same system and principles of operation will have to be carried out, but there must be greater despatch, and inasmuch as a great number of the minutiæ above noticed could not possibly be plotted even by the most careful and ingenious draughtsman, it would be useless and time thrown away to attend to them ; with a scale of 40 feet to 1 inch it is very easy to plot accurately dimensions of six inches, but with a scale of 6 chains to 1 inch, it would be rather trying to the sight to plot one foot which would be the 66th part of a sixth of an inch. A little experience and intelligence will soon teach the practice to be carried out under a variety of conditions and requirements.
On the plan given to illustrate the observations made in this chapter, the centre line of the proposed railway has not been laid down, nor the “limits of deviation," that is, the limits within which it is proposed to deviate, if thought advisable, the centre of the line works before these are actually commenced. Further observations on this subject will be found under the bead of " Levelling," and also in the pages of my work on “Railway Construction."
In concluding this portion of our subject, we may mention a system which is sometimes adopted, when the time by which the survey is to be completed is so short that nothing better can be attempted ; at the best it can only be applied to surveys of a very preliminary character ; nevertheless, where judgment and intelligence have been combined in doing the work, it has often passed the ordeal of “Standing Orders ;” there are besides
sometimes portions of a survey where it is less inapplicable than in others.
In making a survey in this manner, the bases are chained on some distant objects, if any can be selected; these bases are generally short, and the angles of deflection which they make with each other should be carefully observed with the theodolite if possible, or with the box-sextant; at the intersection which the fences make with the base line, the bearings of such fences on both sides of such base should be taken, and the distances from the intersection to the junction, with any lateral fences within about 4 chains' distance on each side; the bearings of such lateral fences should also be taken, and any observations required should be made as to deflections in these fences ; the surveyor should have a competent assistant with him if possible, to make these observations on the lateral fencing, and from whose field notes he must take care to make entries into the field-book as the work proceeds along the narrow slip surveyed ; in such cases as this it is very desirable that the base lines be laid as nearly as conveniently may be along the centre of the work. One considerable disadvantage with such work is, that there is no check even upon the main length of the survey, until the section is taken over the line; a double chain line therefore is very desirable instead of a single one, in which case they would be a few chains apart; these circumstances are mentioned to show the beginner the straights to which we are put sometimes in practice.
When a railway or canal survey is divided between two surveyors, they should, previously to commencing work, decide upon a common base line, which should extend for some length into both portions of the survey ; this line will afford the means of correctly joining the work together.
It may not be improper here to observe, that there are two mistakes generally attending the first attempts at surveying : these are either too much or too little attention to details according to circumstances and the scale of the plan, that is to say, that the surveyor is apt to bestow as little pains on the survey for a working plan as for a preliminary one, or vice versá. To make this plainer, take a plan prepared to a scale of one chain to an inch, and endeavour to reduce this to a plan having a scale of three inches to the mile, and showing every detail on the large plan, it is simply impossible ; again, take the plan of a survey plotted to a small scale, enlarge it to one of eight or ten times the original, take your enlarged copy on to the ground, and you will find your details all wrong. Again, take a field-book of a survey for a large working plan, and endea