A treatise on engineering field-work; containing practical land surveying for railways &c - Softcover
Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1838 Excerpt: ... angle pointed out by the compass-needle, may be also noted at each principal station, which will be a check on the accuracy of the angle; but in the usual description of theodolites, the bearing cannot be read with any degree of accuracy, except by setting the plates and needle at zero, and reading the angle on the limb when the object is bisected, which will be the bearing. Vertical angles are taken for the purpose of reducing lines measured over steep ground to the horizontal measure. On one side of the vertical arc are engraved the requisite divisions for determining the angle, and on the other the number of links to be deducted from each chain's length, to reduce it to the horizontal measurement; but for the purpose of measuring this angle it is necessary to set up a mark at the exact height of the optical axis of the telescope, at the extreme point to be measured to, on which this reduction is to be made, which can then be allowed for in the field in the following manner:--suppose the angle of elevation was found to be 14 30', on the other side of the arc will be found the figure 3 and a fractional part, which signifies that the chain must be lengthened 3 links and a fraction, or that the chain should be drawn forward on the ground that quantity to bring it to the horizontal measurement. In the common operations of surveying it will be found most advantageous to make the necessary allowance in the field, especially where you cross many fences or take a good quantity of offsets, as it will be found very troublesome in plotting to make the necessary reduction for each distance: but where great accuracy is required, the angle should be noted and the necessary reduction made when plotting the work. Thus, on reference to the table for reducing hypothenusal...
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