Sec. 81. Longitudinal and Transverse (Profile) Levelling

The type of levelling described above is from one set up. If it is necessary to ascertain the height difference of points that are con siderable distances apart it is a good plan to

Fig. 108

perform the levelling from several instrument stations, connecting each instrument sta­tion with the adjacent one (Fig. 108).

As the levelling is carried out, points that are common to two ad join­ing instrument stations are termed change (turning) points and the others are called subsidiary points. When levelling special attention must be paid to change points since the error incurred in determining a change point will be passed to other measurements.

When surveying for road construction, levelling is performed to determine the heights of the turning points on a route. It is possible from these to prepare a longitudinal section of the proposed structure. This levelling is called longitudinal levelling. Concurrently with longitudinal levelling it is usual to ascertain the heights of promi­nent points lying at right angles to the alignment, and this opera­tion is referred to as transverse (cross section or profile) levelling.

Before an engineering structure can be erected or a town or vil­lage street laid out, a contour plan is needed. This is obtained as a result of surface levelling.

The levelling needed to design and build an engineering structure is described as engineering levelling.

Sec. 82. Principal Types of Geodetic Levels

Types of level. Geodetic levels are classified according to their precision and method of bringing their collimation plane to a true horizontal position. According to the USSR St.St. GOST 10528-76 geodetic levels are divided into three groups. H-05 type high preci­sion instruments are intended for first-order and second-order level­ling. H-3 type precision instruments are used for third-order and fourth-order levelling. H-10 type engineer's levels are intended for the vertical control of topographic surveys and for giving the eleva­tions needed in engineering surveys.

Depending on the method of bringing the collimation plane to the horizontal there are bubble-type and compensator-type geodetic levels. The former have their telescope's collimation plane brought to the horizontal by means of a cylindrical bubble tube attached to the telescope. In the case of compensator-type levels the collimation plane is horizontalized automatically. The latter, however, require the instrument's axis of rotation to be plumb true, for which a cir­cular bubble level is provided. Compensator-type geodetic levels make it possible, to get accurate staff readings even given small (angles 10-15') deviation of the instrument's axis of rotation from a plumb-true line. They may be used on unstable ground and permit rapid levelling operations.

The first instrument with this design was manufactured in 1945-at ZNIIGAIK (Central Research Institute of Geodesy, Aerial Photogrammetry and Cartography).

According to the USSR St.St. COST compensator-type instruments are designated by the letter K (Russian for compensator), hence H-05K, H3K, H-10K. Instruments that have a limb to deter­mine horizontal angles are additionally designated by a letter Л (Russian for limb), thus H-3KJI or H-10JI.

The main specifications of the geodetic levels currently manufac­tured in this country are listed in Table 10.

Table 10

Parameter	Type of geodetic level
	H-05 H-05K	H-3 H-3K	H-10 H-lOK
R.m.s. error of height difference, mm: (a) per km of a double line of levels (b) at a station for d = 30 m d = 50 m d = 100 m	± 0.5 ± 0.15 ± 0.20	3 2.0	10 5.0
Telescope magnification	40	30	20
Angular value of one division of level tube on instrument on telescope	5" 10"	10" 15"	10" 45"
Mass, kg: of instrument of packing case	6 5	3 2.5	2 2
Operating range of compensator, min	±8	±15	±20

The H-3 (Fig. 109a) and H-10 (Fig. 109b) bubble levels are presently available, in which the telescope and cylindrical bubble tube are rigidly connected and can be moved with respect to the levelling head using a tilting screw. This design has proved to be most effective.

Figure 109a shows an H-3 instrument, while Fig. 109b shows an H-10 instrument. In the H-3 and H-10 devices the image of both ends of the bubble is carried via a prism system to the telescope's field of view (Fig. 110). The observer can thus see both the levelling staff and the bubble's ends simultaneously in the telescope's field of view. The instrument casing on the side of the eyepiece contains four adjusting screws protected with a movable plate. The telescope has internal focusing which can be adjusted with a rack-and-pinion (Fig. 109).

To horizontalize precisely the instrument's collimation plane the level has a tilting (levelling) screw. For coarsely horizontalizing the collimation plane a circular bubble level that is mounted on top of the instrument and can be rotated together with the telescope about the instrument's vertical axis is used. The circular level has three adjusting screws. Pointer sights are for coarsely aiming the telescope. After the telescope is clamped accurate pointing

achieved by manipulating a tangent screw. The tripod head carries three levelling screws. The H-10 device has a ball-and-socket joint instead of a levelling head with three levelling screws. When operat­ing an H-3 or H-10 instrument the collimation plane should be horizontalized

Fig.109

using a tilting screw before a staff reacting is taken. Instruments with a tilting screw and a circular bubble level can be set up one and a half times faster than ones without. The field of view of a telescope with the level's bubble central is shown in Fig. 110. The staff reading is 1500 mm.

Previously, geodetic levels had cylindrical bubble tubes mounted on different parts with respect to the telescope and levelling head.

In all bubble-type geodetic levels it is customary, each time a staff reading is taken, to make the bub­ble central by manipulating the tilting screw or the levelling screw directed along the bubble tube. This ensures that the telescope's collimation line is horizontal when a reading is taken.

Geodetic levels with various types of pendulum compensator are currently manufactured.

Compensators are devices that automatically bring the collimation line or the reading device into a required position. Their function in geodetic levels is to horizontalize the collimation line. Compen­sators operate in different ways. A telescope in operation that is

Fig.111

tilted through a small angle ε (Fig. Ill) will have its cross hairs, from which staff readings are taken shifted vertically, which in turn, displaces the direction of the horizontal collimation line in the vertical plane by a segment

k₀ k₁ = ε f_ob

There are several methods for bringing the collimation line back to its position to permit a correct staff reading:

(1) the cross hairs are brought back from k₁ to k₀ by rotating the reticule about a point P through an angle ε' (Fig. 111a). If P is at a distance l from k₀ along the telescope's optical path, then, obviously,

f_ob ε = lε'

(2) the horizontal ray from the object is redirected at P within the telescope by an angle ε' (Fig. 111b) toward the reticule (k₁). In doing so the equation f_ob ε = lε' must likewise be satisfied;

(3) the position of the horizontal ray from the object from the point of bend P is shiftedparallel to itself until it coincides with the

Fig.112

intersection of the cross hairs k₁ (Fig. 111c). To do this, the condition f_ob ε = lε' = klε or f_ob = kl₁ must be satisfied, where k = is the coefficient of compensation (angular magnification).

Figure 112a is a general view and Fig. 112b is an optical diagram of an H-3K compensator-type geodetic level designated HC-4 by the manufacturer. The instrument consists of the following components: 1—the telescope's objective; 2—a rack-and-pinion to focus the telescope; 3—the instrument's casing; 4—an eyepiece; 5—-a circular bubble level, whose smallest division is 8' to bring the instrument into the operating position; 6—a tilting screw; 7—an infinite slow motion screw for pointing the telescope. The instrument has no clamp. The instrument's compensator consists of two right-angled prisms which, due to the force of gravity acting on the pendulum, as­sume a position in space such that a collimation line striking a levelling staff maintains its horizontal position even if the telescope is tilted through small angles. The air damper rapidly attenuates the pendulum's vibrations and the telescope's collimation line is brought to within 0.5" of the horizontal.