Land and marine surveying by William Davis Haskoll

Author:William Davis Haskoll
Language: eng
Format: epub, pdf
Published: 1868-03-25T05:00:00+00:00

In the same manner, we shall have proof of the angles having been correctly measured, because all the interior angles of the polygon will be equal to twice as many right angles as the figure has sides, less four right angles, or 860**, if the theodolite has been correctly used. There will, however, generally be some two or three minutes' difference, even with the most careful work, unless some particular plan be adopted, such as that which has been mentioned above, of making the readings h\ 10', or 15', &c., by making our marks accordingly. In the description of work to which we have

been more particularly referring, a difference of two or three minutes might be of considerable importance; at least for a working survey amongst buildings we should object to it, and the more so that it may be avoided; but in country surveying, or where looking for such minute accuracy would be loss of time, it is usual to divide the difference or error over the whole work.

As to the care necessary for town-land surveys, we will only point out that the mistake of a foot may be of far greater importance than a foot error in a cutting or embankment on an ordinary section, for in this last case it would only involve some difference as to a greater or less quantity of earthwork; but, serious as such a blunder would be, it would be trifling compared to the setting out works, so that any portion of them might fall a foot within instead of outside of a buUding.

The engineer who has had to set out works through ground thickly covered with buildings, knows all the importance of a correct plan for such purposes.

It is very often by ground plans alone that the lines of working plans are determined—often, indeed, there are no other means of doing it; and not only important and often very expensive workmg drawings are got out entirely according to surveys, and become valueless if these surveys or plans are incorrect, but very great difficulties arise otherwise. Some considerable length of curvCj or straight line, may be laid down on a plan drawn to a large working scale, and properties may be purchased in portions only, and not consecutively along the line, because there may be some cause of delay; indeed, this is a circumstance constantly arising. The works may thus be set out on the ground, and actually commenced, under the guidance of plans only; but if the plans (we mean the ground-plansy and not the working drawings) are incorrect, then the lines will have been incorrectly set ojit on the ground, and very serious consequences mayresrdt. The lines maybe thuslaidout so as to intersect buildings which it was not contemplated to touch, and which we may not even have power to interfere with. When a mass of buildings intervenes between works commenced in two different places along a line, particularly if on a curve, it is often very difficult to check, satisfactorily, work set out from ground-plans only.

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