Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

Fig. 257.—Lister's inclinometer theodolite.

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The instrument is also fitted with a mechanical device for repeating the tangential angles when operating on curves, which obviates the necessity of reading them on the horizontal arc, thus facilitating the work. This will be referred to in the following explanation of the manipulation of the instrument as the "angle repeater."

The main difference between the method of taking cross sections by the level and by the inclinometer theodolite is in substituting inclined bases for horizontal ones, which will be clearly understood by reference to the following diagrams, which illustrate somewhat extreme surface inclination.

Fig. 258.

Fig. 259.

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Fig. 258 shows the levelling method and Fig. 259 the method by the inclinometer theodolite. In the first it will be seen that each section requires to be taken singly with repeated changes in the position of the instrument at each section, involving numerous readings on the staff, booking, and reduction of the levels for changes only. Also the sectional measurements require to be taken in short horizontal lengths with the plumbing of the end of the tape at each length. By the inclinometer method this unnecessary labour is avoided, there being no changes in the position of the instrument, as from one setting a series of sections may be taken on either side of it. There is no reduction of levels, and the sectional measurements are taken on the surface to which the base line is always approximately parallel.

The saving of labour is even more marked in the setting out of slope pegs than in the taking of cross sections, for in addition to the transference of level from the centre pegs to the outcrop of the slope several approximate calculations have to be made before the exact position of the slope peg can be found, while by using the inclinometer theodolite it is only necessary to put in normal slope pegs at intervals of a quarter of a mile, or at such distances apart that a ranging rod may be seen from one point to the other, and by setting up the instrument at each alternate peg, or at half mile intervals, the whole of the intermediate pegs for a quarter of a mile on each side of it can be "boned" as simply as ranging a straight line, the telescope being inclined to, and revolving in the plane of the slope. In this manner as much work may be done in a few hours as will take a week with the levelling method, and this without the slightest physical or mental strain to the operator.