TYPES OF LEVELING OPERATION

TYPES OF LEVELING OPERATION

(1). Simple levelling: When the difference of level between two points is determined by setting of levelling instrument midway between the points, the process is called simple levelling.

   Suppose A and B are two points whose difference of level is to be determined. The level is set up at O. After proper temporary adjustment the staff reading on A and B are taken. The difference of level between A and B (Fig. - L.10).   

(2). Differential levelling: Differential levelling is adopted when (i) the points are a great distance apart, (ii) the difference of elevation between the points is large, (iii) there are obstacles between the points.

   This method is also known as compound levelling or continuous levelling. In this method the level is set up at several suitable positions and staff readings are taken at all of these.

   Consider Fig.- L.11. Supposed it is required to know the difference of level between A and B. The level is set up at points O₁, O₂, O₃, etc. After temporary adjustments, staff readings are taken at every set up. The points C₁, C₂ and C₃ are known as change points. Then the level between A and B is found out. If the difference is positive, A is lower than B. If it is negative, A is higher than B.

   Knowing the RL of A, that of B can be calculated.     

(3). Fly levelling: When differential levelling is done in order to connect a bench mark to the starting of the alignment of any project, is called fly levelling. Fly levelling is also done to connect the BM of any intermediate point of the alignment for checking the accuracy of the work. In such levelling only the back-sight and fore-sight readings are taken at every set up of the level and no distances are measure along the direction of levelling (Fig. – L.12). The level should be set up just midway between the BS and the FS. 

(4). Longitudinal or profile levelling: The operation of taking levels along the centre line of any alignment (road, railway, etc.) at regular intervals is known as longitudinal levelling. In this operation, the backsight, intermediate sight and foresight readings are taken at regular intervals, at every set up of instrument. The chainages of the points are noted in the level book. This operation is undertaken in order to determine the undulation of the ground surface along the profile line (Fig. – L.13).

(5). Cross-sectional levelling: The operation of taking levels transverse to the direction of longitudinal levelling, is known as cross-sectional levelling. The cross-sections are taken at regular intervals (such as 10 m, 20 m, 40 m, 50 m, etc.) along the alignment. Cross-sectional levelling is done in order to know the nature of the ground across the centre line of the alignment (Fig. – L.14).   

(6). Check levelling: The fly leveling done at the end of the day’s work to connect the finishing point with the starting point on that particular day is known as check levelling. It is undertaken in order to check the accuracy of the day’s work (Fig. – L.15).

(Next post on “PRINCIPLE OF EQUALISING BACKSIGHT AND FORESIGHT DISTANCES”)

DIFFERENT TYPES OF LEVELS

DIFFERENT TYPES OF LEVELS

(1). The Dumpy Level: The telescope of the dumpy level is rigidly fixed to its supports. It cannot remove from its supports nor can it be rotated about its longitudinal axis. The instrument is stable and retains its permanent adjustment for a long time (Fig. - L.3 and L.4).

Fig. L.3 

Fig.- L.4 

(2). The Wye Level (Y-Level): The telescope is held in two ‘Y’ supports. It can be remove from supports and reversed from one end of the telescope to the other end. The ‘Y’ supports consist of two curved clips which may be raised. Those the telescope can be rotated about its longitudinal axis (Fig. - L.5).

 Fig.- L.5

(3). Cook’s Reversible Level: This is a combination of the dumpy level and ‘Y’ level. It is supported by two rigid sockets. The telescope can be rotated about its longitudinal axis, withdrawn from the socket and replaced from one end of the telescope to the other end (Fig. - L.6).

 Fig.- L.6 

(4). Cushing’s Level: The telescope cannot be removed from the sockets and rotated about its longitudinal axis. The eye-piece and object glass are removable and can be interchanged from one end of the telescope to the other end (Fig. - L.7).

Fig. - L.7 

(5). The Modern Tilting Level: The telescope can be tilted slightly about its horizontal axis with the help of its tilting screw. In this instrument the line of collimation is made horizontal for each observation by means of the tilting screw (Fig. - L.8).

Fig.- L.8 

(6). The Automatic Level: This is also known as the self-aligning level. This instrument leveled automatically within a certain tilt range by means of a compensating device (the tilt compensator).  telescope can be tilted slightly about its horizontal axis with the help of its tilting screw. In this instrument the line of collimation is made horizontal for each observation by means of the tilting screw (Fig. - L.9).

Fig.- L.9

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