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It may perhaps be thought that too much stress has been laid upon forms, in the above description of the details of an extensive survey; but method is a most essential part of an undertaking of such magnitude; and without excellent preliminary arrangements to insure uniformity in all the most trifling details, the work never could go on creditably. In topographical surveys on a smaller scale, where the boundaries of parishes, &c., are not to be shown, or the contents of various portions to be calculated, the same rigid attention to minutiæ is not requisite; but before closing this branch of the subject, it is only necessary, as a proof of the mass of valuable statistical and geological information that can be collected during the progress of a national trigonometrical survey, and which is quite out of the reach of any individual, to turn to the first volume of "The Ordnance Survey of the County of Londonderry." If this valuable accompaniment to the field operations could have been continued throughout every county, Ireland would be possessed of more available local knowledge than is on record in any part of the world.
The following brief hints may be found useful in filling-in the detail of a survey with the chain and theodolite.
The field-book should be kept in ink in the field, and have a distinctive letter marked on it as a reference; every day's work should be dated, and the names of those employed entered. On an extensive survey it is also necessary that every book should be kept on precisely the same system, that one person might find no difficulty in plotting from the book of another.
The theodolites should be constantly examined and adjusted, and the chains compared every day with a standard chain, or marks laid down from one for that purpose, and their errors, if any, either corrected or entered in the field-book, to be allowed for in plotting. The offsets should be numerous, and minute in proportion to the scale upon which the survey is to be plotted*, and the names of all
* From one to two chains should be the maximum length of offsets where the contents of inclosures are to be computed, or even laid down on a large scale. These limits must of course be extended in filling in the interior in less accurate surveys, or which are to be plotted on a very small scale. As drawing-paper is very much stretched when mounted on a board, and partially contracts when cut off, and as it is always liable to change from the
towns, villages, &c., carefully noted, and care taken to insure their correct orthography, and to quote the authority upon which it rests when different from that sanctioned by custom.
In measuring long lines between conspicuous objects, marks should be left, to be connected by check lines, or on which to base smaller triangles; where impeded by a house or any obstacle, the means of avoiding it and returning again to the measured line are to be found further on.
Irregular inclosures and roads, even where triangles cannot be measured, can still be surveyed by the chain alone, but of course not so accurately as with the aid of the theodolite.
This method of "traversing" is managed as follows:-Suppose
A B the first line, and B C the direction in which the next is required to be measured, prolong A B to E, make B F equal to B E, and measure the cord E F, from which data the direction of BC. can be laid down.
The dimensions in the field-book may be kept either between two parallel lines running up the page, with the offsets written on the right and left of these lines as in the example facing page 36, or on a species of diagram bearing some sort of resemblance to the outline of the ground to be surveyed, which latter method is supposed to assist in the plotting; but if references to the starting points of the different lines, and their junctions with each other, are entered in the field-book kept according to the first system, and the angles forward written on the right or left of the ruled lines according to the direction of the next forward station, there can never be any difficulty in plotting the work, even after a con
atmosphere, it is a good precaution to divide the scale for laying off distances from the fieldbook, on the paper upon which the plot is to be made, as it will then always expand and contract with the outline of the survey; and also to mount the paper before commencing plotting, or not at all.