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).

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