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

The operation when the line is straight is to set up the instrument at a centre peg some distance away from the normal in the manner previously described, viz., with the front legs set across the centre line, the back leg on the centre line, and the bubble on the tribrach set level before adjusting the instrument, which manipulation produces a perfectly vertical tilt.

After adjustment, set the horizontal arc to zero and direct the telescope to the centre line, clamp the lower limb, set the vertical arc to the angle of the slope, and bring the horizontal arc round to an angle of 90°, or a right angle to the centre line. Liberate the telescope and direct it again to the centre line, and by working the back adjusting screw tilt the instrument until the cross web intersects the top of the centre peg at the normal. The telescope will now revolve in the line ag parallel with the plane of the slope and at a distance of 21·60 feet from it. Therefore any point on the surface in the line of the slope where 21·60 can be read on the staff is the outcrop of the slope and the position of the peg.

In this example, when the required reading is higher than the ordinary staff, lower the tilt and take an intermediate reading, as at f in the diagram, Fig. 263, which may read, say, 12·00, when the required reading on the peg will be reduced to 9·60.

Fig. 263.

Larger image

In setting out normals on a curve by this method the only difference in the operation to that above described is that instead of in the first instance clamping the instrument with zero on the centre line, it must be clamped with zero on the chord line, i.e., at double the tangential angle for the distance the instrument is from the peg, as before explained in detail for operations on curves.

By this method two normals may be set out at least 20 chains apart from one setting of the instrument, or several slope pegs may be set out in like manner, which under certain contingent difficulties of ground surface is an advantage of considerable importance.

In this connection there is also an alternative method of putting in the slope pegs after the normals have been set out, which, under certain conditions, may be employed with advantage.